Most high street stores sell separate tops and bottoms now. River Island and Oasis always seem to have a good selection. Topshop might be a good choice as they come up small. I have some lovely bikinis from Tesco . You do have to sort through some crappy ones there but they usually have a couple that are better quality fabric and cut. Boden also have separates and their chic bikini is gorgeous and very flattering. If you can cope with sorting through a few then TK Maxx is also a good supply. I have a Michael Kors bikini that I snapped up for 15 quid 6 years ago and it still looks new.