I agree with the concept but not the specifics. Your internal format should match what is used on UK landlines and mobiles.
On an incoming call from London, you would change +442072345678 to 02072345678. On a call from Paris, you would change +33123456789 to 0033123456789. If the called number comes in as +44113334455 it would be changed to 0113334455. These changes would be done with a custom context similar to from-pstn-e164-us, but modified for UK.
On outgoing calls, you would define [macro-dialout-trunk-predial-hook] to rewrite caller ID. See
Rewriting the outbound number can be done in the Dialed Number Manipulation Rules for the trunk.