Here are a few side dishes that I like to have. I recommend using organic veg if possible as the flavour is much stronger and fresher, though to be fair it doesn't keep quite as long.
Mashed potatoes
If you're cooking for about 4 of you, use about 1.3kg potatoes if you want to make sure everyone has enough. I always use Maris Piper potatoes, although I also hear good things about Desirees and King Edwards. Wash and peel them, then cut them into roughly even sized pieces (about the size of a golf ball). Put them into a saucepan with enough cold water to cover, add salt and bring to the boil. Turn the heat down a bit so that they simmer briskly rather than letting the water flow all over the place for about 20 minutes. To check if they're done, skewer one with a sharp knife - if it slides off easily, then they're done. Drain the water out and put the saucepan back on the ring on a very low heat for about a minute so some of the water can evaporate. Mash up with a potato masher (well, what did you expect to use, a tennis racket?) and add about 50g of butter. Heat a small drop of milk in the microwave and CAREFULLY add just enough to the potatoes to make the mixture smooth but not sloppy. Add salt and pepper to taste. Personally, I like to add a bit of English mustard, but some people add spring onions, herbs, cheese or even apples (the latter goes well with pork).
Mashed potatoes
If you're cooking for about 4 of you, use about 1.3kg potatoes if you want to make sure everyone has enough. I always use Maris Piper potatoes, although I also hear good things about Desirees and King Edwards. Wash and peel them, then cut them into roughly even sized pieces (about the size of a golf ball). Put them into a saucepan with enough cold water to cover, add salt and bring to the boil. Turn the heat down a bit so that they simmer briskly rather than letting the water flow all over the place for about 20 minutes. To check if they're done, skewer one with a sharp knife - if it slides off easily, then they're done. Drain the water out and put the saucepan back on the ring on a very low heat for about a minute so some of the water can evaporate. Mash up with a potato masher (well, what did you expect to use, a tennis racket?) and add about 50g of butter. Heat a small drop of milk in the microwave and CAREFULLY add just enough to the potatoes to make the mixture smooth but not sloppy. Add salt and pepper to taste. Personally, I like to add a bit of English mustard, but some people add spring onions, herbs, cheese or even apples (the latter goes well with pork).
UPDATE FROM TIM: a new method I use to make mash is to cook the potatoes whole with the skins on and then peel them once they've cooked. It takes longer (up to about 45 minutes, depending on the size of the spud) but it keeps all the starch and flavour in.
Roast potatoes
The daddy! Peel, chop and boil the potatoes as above, but this time only for about 5 minutes - don't cook them completely yet as they will just fall to bits in the oven otherwise. In the meantime, heat the oven to 210 degrees and put a tray with a splash of olive oil or goose fat in so that it heats up. When the potatoes are done, drain the water as before (although you may want to reserve it to make gravy) and let the water evaporate a bit. Next, get a fork to fluff up the edges and sprinkle some flour over them - this will make them go nice and crispy in the oven. Put them in the tray and roast them in the hot oven for about an hour and a half, turning every half hour or so to ensure an even coating of crispy awesomeness. For a bit of extra flavour, try adding garlic cloves and rosemary leaves.
Potato wedges
I'm no health freak by any stretch of the imagination, but I don't like deep frying things if I can help it and since these wedges are very tasty without the mahoosive fat content I don't see the point of making normal chips. Put the oven on at 210 degrees. Wash your potatoes, but don't peel them. Cut them in half, then half again, then half again until you have 8 chips. Boil them in some water for about 5 minutes (no more) then put them on a baking tray skin side down (this helps them to cook evenly), season them, drizzle them with olive oil and put them in the oven for about half an hour until they're golden and crispy. You can add herbs to this for added flavour; the last time I made them I used rosemary sprigs and smashed garlic cloves, which turned out rather nicely!
Yorkshire puddings
For my non-English readers, Yorkshire puddings are basically just big puffs of batter served alongside a roast. They should be crispy on the outside and soft in the middle to soak up the gravy. To be honest, the best recipe I've tried for this is James Martin's, which can be found at online at http://www.bbc.co.uk/food/recipes/database/yorkshirepudding_81824.shtml. If you don't have dripping like he recommends, or you want vegetarians to be able to eat them, then use sunflower oil, vegetable oil, groundnut oil, or basically anything that has a high smoking point (olive oil or butter will burn like last night's curry so don't even think about it!). Also, don't worry about a little bit of smoke while they're cooking, though obviously you should use some common sense!
Braised cabbage
This is a much tastier alternative to the school-dinner style crap that's been boiled beyond mortal comprehension. I like Marcus Wareing's recipe for it that he made on the BBC series "Great British Menu" http://www.bbc.co.uk/food/recipes/database/lancashirehotpotwith_81803.shtml, although when I made it I used a couple of tbsps' Worcestershire sauce instead of bay leaves because I didn't have any to hand and it turned out fine. Some people use cinnamon and/or whole cumin seeds when they make this, so see what you prefer. This side dish goes particularly well with roast pork, in which case I recommend using white wine vinegar or cider vinegar instead of red wine vinegar, depending on what kind of gravy you're making.
Honey roast carrots
This is another alternative to school-dinner vegetable annihilation. Trim the carrots by cutting off the tops and the very bottom tips, then peel them. Put them in a saucepan of cold water and bring to the boil. Boil them for about 5 minutes. You can cut them up first if you like but personally I don't bother, I just chuck them in whole! Next, heat a bit of olive oil with a knob of butter at a medium-high heat and put the carrots in, adding a generous glug (technical term there) of clear, runny honey. Season with salt and freshly ground black pepper and cook for about 30-40 minutes, turning frequently to prevent the honey from burning too much (it will happen a bit but that's unavoidable, it doesn't matter as long as it's only a little). Serve garnished with fresh parsley leaves.
Plain boiled rice
Obviously this is more for Oriental food than British, but on the other hand, oriental food (or at least food inspired by it) is very popular in the UK - did you know that chicken tikka masala is actually Scottish? You want approximately 75g of rice per person if it's part of your main course. Soak it in water for about half an hour before you start. Cook it in 1.5 times as much water as rice; bring it to the boil, then turn the heat right down and cover and simmer it for about 10 minutes. Drain any remaining water, then fluff it up with a fork and leave it to rest for another 10 minutes before serving. You can mix in a little bit of freshly squeezed lemon juice as an optional extra.
