I don't want to worry you and to be honest, i'm not 100% certain on this rule (clarification from a
bona fide MoT tester would be good please!) but i'm fairly sure welded repairs to sub-frames are a fail point for the MoT - thing is they need to see it's been welded to fail it and judging from the rest of your work that could be nigh on impossible!
I've never understood this rule (if in fact it is a rule) but suspect it dates back to the era of the Mini (the real one, not the BMW thing that is now the antithesis of its name) where rear subframes would regularly rot faster than a Fiat in a saline solution. Repairs were often made using pigeon poo and putty and usually only lasted long enough to get an MoT and sell the death-trap to some poor unsuspecting mug. Said mug then drives said Mini as if they're an extra in The Italian Job only to find the subframe departs from the body leaving them in a very unenviable state!
That said, if the weld repairs are done properly i see no reason why the repair shouldn't be at least as strong as the original metal.