Setting the barrel back prior to rechamber/rebore would require moving the forend latch forward the same distance on the barrel which is more work and $$ and then you will still end up with gapping all around the barrel and forend.
I hate to suggest it, but seems like a .350 of some sort with a bigger case than the 318 would be needed to clean everything up.
As mentioned, a 350 Rigby would be something if it can be squeezed into the action and magazine (I think it could especially if you chose a shorter OAL than the original)but maybe a 350 on the 375 Ruger case would get you there even if not a classic British chambering. The 350 Ruger would be pretty much a 35 Newton which is a great cartridge.
This is one of those situations where you either grit your teeth and sell the rifle (which may be at a loss) or decide it will be your custom rifle for the rest of your days and make it into something you can use and get to know so to speak, without worrying about its value to anyone else.
Lesson learned...
I think most of us have a couple rifles like that - I know I sure do!