We've used Voiptalk from Teleappliant at work for that last few years and they seem pretty reliable and their pricing seems reasonable for a decent quality VoIP provider. You can sign up for free to try the service out (you appear to get an 056 incoming number these days, it used to be an 0871 one, and can buy call credit to make PSTN calls and make VoIp calls for free).
I've also heard Gradwell are OK although I think they're more targeted at the business market.