Tesco Internet Phone
Company Background: UK supermarket behemoth who sell a service provided for them by Freshtel. As far as I can tell, as Freshtel don't offer a service directly in the UK, this is the only way to use them.
Tesco offer inbound and outbound PSTN services, with choice of a free UK geographic phone number (from an incomplete but fairly comprehensive selection). Not the easiest of services to sign up for (complete pain actually) but works well once done, and the UK Anytime package seems a fairly good deal for those wanting a simple SOHO second line via VoIP (my use case). Customer service helpful and friendly but based in India, so can be heavily accented.
Allocation of an 0870 number: N/A
Allocation of an 0845 number: N/A
Allocation of UK geographic number: Free
Incoming calls from PSTN: Free (caller pays normal PSTN rates)
Monthly Subscription: either Free (with PAYG for outbound PSTN calls) or various packages that include "unlimited" calls (subject to a fair use clause) to various territories.
Outgoing calls to another VoIP/SIP user: Calls to other Tesco Internet Phones are free
Outgoing calls to PSTN: pre-pay credit required, or to have registered a credit card / direct debit mandate. Rates seem reasonable: 2p/min in UK, 1.7p/min to top 30 international destinations. Non-geographic terminations are obviously much more expensive. See
http://www.tescointernetphone.com/about/rates for details.