@Kilgeever53 Great news, ive finally had it fixed!
I'm now getting 926mb down and 107mb up.
After trying a new ONT (that didn't fix it) I called BT and insisted that they move my connection to a different VLAN, they'll push back and say "it won't be that, its only at 30% capacity, its not a HOT VLAN etc etc" but tell them that others in the same situation, also without being on hot VLANS, had there's moved anyway as a last resort and its worked.
I hope this helps and that if you manage to get them to agree trying this, it fixes your issues!
Ian
@Kilgeever53 Hope you get well soon, good luck with everything.
Ian