I don't know of any DID provider with free geographical UK numbers.
If you want
non-geographical DID (sort of 0844 or 0845 numbers) you can try
VoipUser service (they offer free non-geographical numbers). If you are able to pay a little value for such DID (for around € 2.95 a month) you can get it through
MyDivert service, which is very good and they have all UK area codes (I'm a pleased MyDivert customer).
Regards,