If the lock is operationable and the barrel condition is Ok, I'd say 100-200 dollars? Really, I'm looking at the potential of a shooter peice, not a collectable piece. The gun is not complete and the condition is not fully known. I hope this helps, and if anyone feels I am off mark, please chime in with your estimate.
"London" doesn't always mean anything, Belgian makers often put the London mark on guns that never saw Britain but the proof marks and "14" do sound English, 14 gauge. Can you post pics of the proof marks?