How much should you spend on wine?
If you see a wine that "was £12" and is now "just £8", chances are that it's only going to taste like a £8 wine. There are exceptions of course, but stick with this rule of thumb and you'll never do your dinner an injustice
If you're not vegetarian or vegan, then the meat will certainly be the most expensive part of the meal. In fact, you could easily just use that cost as your benchmark for the wine. Take a look at the leg of lamb below; £18.49.
There are 6 decent sized glasses in a bottle (125ml each), so if you really wanted to push the boat out, seek out the best you can find. However, these days there are few wines at that level on offer if you're a high street wine shopper.
If you're going to have a traditionally-styled roast lamb, you could pick a good Rioja in the £10-12 range. Then use the £6-8.45 left for a white as the aperitif.
Of course, if you added the veg as well, you'd be well north of the £20 budget for wine, which leads into really interesting wine territory (though tread carefully).
Roast chicken sees us in different circumstances all together; £8.62 on the meat, plus a few for veg, allows £10 or so to pick something good.
Traditional wisdom would have you looking for a nice Pinot Noir, if you prefer red (New Zealand or Chile for a fruitier style; France for a more savoury, classic rendition) or Chardonnay for white (Burgundy from France is unlikely to wow at this price; but Australia, South Africa and New Zealand would be good options).
Alternatively, you could just say no to that and look for a really good Riesling instead.
Whatever your decision, you see the point. If you're prepared to spend X amount on the food, then why defer to the lowest common denominator when it comes to what to drink with it.
The food ingredients are just part of the meal; wine deserves just as much thought and attention. After all, you wouldn't go to all that trouble just to drown the dish in tomato ketchup, would you?