I know that Andrews & Arnold have geo numbers in some area codes and charge £1.15 exc VAT per month. You can receive calls via the internet (VoIP) for no extra cost. Alternatively they do 033x numbers for the same price:

I haven't used them, although I think there are some on here who have. The point is that you won't incur any "per minute" charges of routing one 01/02/03 number to your existing 01 number.

If you're willing to go the VoIP route, there are multiple providers that will give you a free geographic number.  sipgate.co.uk wil do so, and you can get Tesco Internet Phone pay-as-you-go accounts for free too.   Just pick a address in the area you want a number in...

All you need is an old PC, go download a pre-packaged version of asterisk, e.g. the free version of trixbox, and install it.  You can then set up as many trunks (incoming or outgoing accounts) as you want.  Then find a SIP aware phone e.g. some nokia mobiles, or adapter for an existing POTS phone, and point it at your asterisk box, and you're away.. 

I use A&A for my 03 numbers, and have never had a problem, although I'll admit they are very very lightly used.