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.