In my experience, O2 speeds are always terrible. Try EE.
O2 are heavily upgrading, depends where you are. They have less spectrum (ie, paid less!) than Vodafone or EE, but they have a LOT of users.
VirginMedia Cable M200 22/nov/19
7 years of FTTC VDSL BT then plusnet with sync from 55/12 to 46/5
20 years of broadband connectivity since 1999 trial - Live BQM