It's a problem caused by the enforced competition rubbish we have, whereby there isn't really competition just loads of companies reselling one of the companies' products.
FTTP is a tiny proportion of the rollout, but is available to ISPs at the same prices as FTTC, with almost identical ordering systems etc. The differences are purely technical.
So, instead of a VDSL router, and no Openreach modem, as most ISPs go for these days, you need a special openreach modem (not an FTTC one) or "ONT". You always get the promised speed (the max sync for FTTC) so even if you order the lower FTTC matching products, you will get exactly 40/10 or 80/20 no matter what.
These other ISPs go "Hmm, but this is a tiny proportion of the market, and means training our staff to deal with a different set of calls. Let's just not bother" and suddenly BT is your only FTTP isp.