No7 have a handheld device that will shade match you. They'll try it on and make sure you're happy with it and there's no obligation to buy from their range. The most a foundation from them costs is around £25 but I got a fab one for under £15 with medium buildable coverage. Despite being very pale I wasn't even the lightest shade match!
Otherwise MAC are very good for matches and their foundations are under £30 if you're after light/medium coverage.
A medium coverage is best then you can conceal or build it as required. Most pale shades seem to wash me out too and I rely on a good blush and bronzer to contour and give my face colour and definition.
They'll ask how often you wear make up usually, if you have a preferred type (liquid, gel, mousse, powder etc) and your skin type (normal, dry, oily, combination). They should also ask what coverage you'd like and whether you prefer a natural glow or matte finish.