I am not sure that that is the case, my ISP seemed to think it was okay to attach 3rd party kit.
I was recently testing another VDSL2 modem that is not yet on the UK market and it CONSISTENTLY synced 12% slower than the BTO modem. I was surprised at the loss as the manufacturer has a very good name.
It was not a profile related issue, I could swap the modems at will and alter my sync. Not all modems are created equal so make sure you buy one from a retailer with a generous returns policy - you may find it slower than the BTO one.
PS Don't ask for details of the kit. If I told you I'd have to shoot you; then they would have to shoot me .
Stress - the condition brought about by having to resist the temptation to beat the living daylights out of someone who richly deserves it.