I've bought them a few times - when I built my house in Aberdeenshire the budget was getting tight and I was allowed the VAT back on landscaping including plants so I went on a shopping spree at Morrisons down the road and planted out the whole garden(half an acre) including 4 apple and 2 plum trees for £70, got £14 of that back from the nice VAT man, and a good few apples in the first year. When I sold it 2 years later there were 25 apples and 5 plums.
When I moved down here 2 years ago I planted some more and they had teensy wee apples last summer, so maybe I'll get some better ones this year (provided the roots don't get waterlogged again with this weather).