Most of the speed you get is completely dependant on the Openreach equipment. You'll be stuck with that even if you change ISPs.
If there's a line fault that can be addressed get it dealt with.
I was getting around 55 down once they fixed the previous fault, then since Jan its been going down and now under 36. Also last time when doing a BT test it was showing I was on a lower profile which seems to be the case again. I don't know if this happens automatically for some reason or not. BT speed check doesn't show me diagnostics keep getting and error on the website.
Can you not get FTTP? Most new builds have FTTP installed now...