A groupset is the term used for the drivetrain/brakes, on a road bike, that the chainset (crank), shifters, brake levers (or STI), F&R mechs (derailleurs), brakes, cassette & chain.

For the requirements you have already given, you don't need a full groupset, you will need a rear mech, crank, downtube shifters, brake levers & brakes, if you want to go with 10 speed, would be looking at Tiagra for the rear mech, cassette, Dura Ace for the dowtube shifters, as they are you only real choice, KMC for the chain, Tektro for the brakes & levers. The crank could be an issue with the budget, something like this http://www.spacycles.co.uk/products....=m2b0s109p1698 you would need to research what T the chainring should be & what Bottom Bracket.

For wheels this will most likely push the budget to £3-400, would be looking at low end, but not budget, something like Shimano R500/501, or Fulcrum 7's.

What's the state of your bars, seatpost, saddle etc have you accounted for these?

For where to buy, some bargains can be found on ebay, but you will have to be patient, and be aware of the postage charges, for shops, Merlin, Ribble, Wiggle, Chainreaction among many others, and don't discount the German suppliers, Rose, Bike Discount, Bike 24.