I think that this depends if you are either proofing the gun for safety or for the future value in the UK.

If it is for safety then please see my contribution under the proofing sticky.
Then it is not worth the trouble, you could prove the safety of the gun to your own satisfaction using the same method..

If the gun is going to be a classic masterpiece then perhaps for asthetic reasons then London proof marks would be worth the expense.

I know an RFD who is also a friend that would and could handle the gun for you to get this done but the choice to spend this money must be yours.

Regards