These people look like a possibility. Case something like 35 USD, shipping to the UK roughly 17 USD... (you'd risk getting hit with VAT too).

http://www.mightypouch.com/madetomeasurecasesolutions

They do have a 'Custom solutions' section, where you could no doubt get clever and add a flap to slide the keyboard out of a flip case. But it'd be complicated, and I imagine they wouldn't do it cheaply unless a lot of people wanted them.