Good luck, all baby clothes found on the high street seem to be made cheaply abroad. Your best bed would be searching a store that isnt part of a chain, e.g. vanilla bloom. Try contacting them and seeing if they have any clothes made in the uk.
I agree about Vanilla Bloom making lovely clothes, I wouldnt know whether or not they sell british made clothes though. To be honest I think your best shot is to look at designer baby and kids clothes. Scarlet Sky do some lovely clothes, and they sell brands like Hackett London and Joules who are as British as you can hope to get.