Got a few older cards and have been actively collecting some of the newer sets since last year (2011 Topps Series 1,2 and Updates, 2012 Topps Series 1, Heritage and Gypsy Queen so far....)
Buying single cards in the UK the best places are probably ebay or checkoutmycards.com. There's not a lot to be found on UK ebay but a lot of US ebay sellers are happy to ship to the UK.
Buying unopened packs / boxes I've been using dacardworld.com. Their shipping isnt too bad, especially if you make sure you order a few things at a time. You have to beware import VAT though (20%) and customs handling costs. You can also find a few packs/boxes on UK Amazon but they tend to be more expensive than buying from the US as the seller has had to pay all those costs already to get them into the country.
Going to New York and Washington DC in just under a months time so hoping to pick up some more cheaply then.
A good internet source of info on the different newer sets is baseballcardpedia.com