The way it works in the northwest is typically it'll be a pimp/courier driving a BMW with two or three girls in, based near a major motorway junction e.g. Warrington, Preston, Bury.
The pimp will be running up and down the motorway dropping girls off and picking them back up again, mainly from hotels though some will do homes as well - as long as they're close to major road access.
Sometimes the girls will be offering additional distribution and services of other illegal products, not just sexual.
In London something similar will be happening, though they'll be based close to major roads rather than motorways