I think they all use drivewealth. I think its also the reason a lot of them cant support transferring US shares.
HL for example (and others like Equiniti, Computershare etc.) as i understand it hold foreign shares in a CREST Depository Interest. This as i understand (someone might correct me if im wrong) is what allows for foreign share to be handled in the same way UK shares are, and what facilitates the ability to transfer foreign shares between brokers.
Since drivewealth has no link to CREST, theres no way for you to transfer them or manage them in the same way. its another differentiating factor that new brokers are unable to handle unless the move away from drivewealth
(i could be off base, but thats my current understanding)