I know from experience (I co-own superiorproxy(dot)com) that there is definitely a market for this kind of service. We only provide US proxies, but I've had a number of users request international IPs - Canada, south america, the UK, etc. Especially the UK.
If you do it right and don't screw your clients, you'll definitely see some interest. Hell, let me know if you have anything in the UK!