The problem with your fuel tank is that it is way too early to be on there, unless you are just building an art project. The fuel tank is ~40 pounds of dead weight load, it's not a frame member, it adds stress to the chassis not strength and it's useless if the engine doesn't run.
How pretty the metal polishes up would be no surprise and the least of my concerns, if your engine doesn't crank over with healthy tested compression you are looking at an engine rebuild and that's not an easy engine to work on. You live in Britain, don't they have mandatory vehicle inspections there? The Honda GL/CX 500 had marginal adequate frame strength before you took an angle grinder to it, although I have seen worse, I seen one where the guy cut away even more of the frame thinking the engine is there to hold the wheels on. ... Fact is that engine is going to do everything it can to go in a different direction then the rest of your bike and the part you hacked off does a lot more then provide somewhere to mount a saddle. If your goal is to build yet another fenderless rolling indoor art project it will work just fine, if you are building a motorcycle for a human to actually ride I would fear for their survival, although I suspect the chance of such a motorcycle ever seeing road insurance, a license plate and being ridden to any extent is slim to nil. Best luck with your art project Dan