Another vote for Halfords (the Advanced range anyhow); there's generally good deals to be had there. Try also local motor factors; they often have quality kit at decent prices. B&Q are also worth a look.
I'd try and get a set from 8mm-19mm (probably most will be anyhow), as it'll give extra flexibility if you should need it, and a decent socket set of the same.
If you're just starting out, there are some Mechanic's Toolkits / Car Repair kits, which include things like screwdrivers, pliers and Allen keys, as well as spanners / sockets.
As already stated, buy decent stuff to start with, look after it and they'll look after you, unlike cheap stuff that gets used once or twice, breaks, and gets binned.