If you are able to take G.Fast, then this means you are reasonably close to the cabinet - probably no more than 300m.
However, G.Fast performance drops off very rapidly with distance, and especially upload performance can be worse than VDSL when you get close to the limit.
Therefore, if upload performance is important to you, you will be better off with VDSL. Being reasonably close to the cabinet, you should get a rock-steady 80 down / 20 up. If you want more, you can buy two lines and bond them together (although not many ISPs offer that - AAISP is one).
As others have said, switching ISPs isn't going to make much difference, when the underlying problem is the Openreach "last mile" link to your house, because they all use the same link.
If you've been told that you'll get FTTP in the next 6 months then of course this issue goes away entirely: FTTP "just works" and doesn't have any variation in speed with distance from the head-end exchange (it bypasses the FTTC cabinets completely). The overall quality of service will still depend on the quality of the backhaul and Internet transit provided by the ISP, so don't expect the cheapest ISP to perform as well as the most expensive.
Do also be aware that some people have been told that FTTP is coming, only to have plans delayed or put on hold indefinitely, when either construction difficulties or budget changes have taken place.