Hi Pete,
Please don't block access to foreigners. You can set the geo-target in Google webmaster tools account, if you have not already done so. It is completely normal with country-specific websites getting traffic from all over the World. A small or probably 1 in a thousand foreign visits might come from UK people traveling abroad. So to conclude, its my honest opinion not to block access to foreigners even the bounce rates might be alarming. Its common and natural and search engine like Google know about this pretty well and there won't be any issue to your website from their end.
Best regards,
Devanur Rafi