Judging by what you've made us aware of on the forum before, perhaps there are better things to be spending money on. I'm just thinking about your interests.
I can understand that your VDSL connection has taken a bit of a hit, but beyond the stats do you actually notice the difference? If you do, then I think you have a reasonable reason to switch to G.fast or VM.
I don't have firsthand experience with VM but I think their network has improved. I think the M350 (£43/m with phone) might be a bit overkill, however. It might be better to start off with M100 (£33/m with phone, 10 Mbps upstream), or M200 (£38/m with phone, 20 Mbps upstream) if you need more upload speed. Those prices are for 12 month term. Don't forget about cashback at TCB or Quidco!
I don't think G.fast is a bad option, but there is an uncertainty about the upstream speed. VM might be a bit less hassle in your case and a certainly more flexible for speed options. I really don't think the upstream will be as bad as the DSL checker says, but there is really no way to know until it's live - and by the time it's live, the cooling off period of ordering will be over.
I think it's another case where I don't think you need to change anything, but that's the view of an outsider. My view is that VM is probably a better choice in this scenario due to the flexibility in contract lengths (so you can try it first if you want), various packages, and quite competitive prices compared to the G.fast alternative.
Edit: Fixed pricing error (was on mobile, sorry)