The weird and wonderful laws and customs around selling sex in this country, what is legal and what is illegal but ignored, must really confuse those not used to punting here. It must appear bizarre that the agencies don't list intercourse, but do list a level, OW/OWO, etc.
Traditionally, escort agencies didn't list any services, as you were buying the girl's time, and they often stated that what went on between two consenting adults was nothing to do with them. That way, the agencies could claim that they were not breaking the law, as they were not advertising sex for sale. There are still many agencies that don't list services, for this very reason. However, even with such agencies, it is not uncommon to see highlighted those girls who are "adventurous" or "full service", as a euphemism for saying they offer A level.
Some agencies have moved on from these euphemisms to being quite explicit about what non-standard services each girl offers, over and above the unmentionable intercourse. However, I can remember some London agencies who explicitly listed A level being shut down, as this is patently a sexual service. For a time, almost all London agencies stopped advertising explicit services. But things seem to have relaxed again. Until the next clamp-down?