My conclusion so far, the girls will charge extra if:
1. They think they will get more business and thus make more money if they structure their price with "basic + extra". This could be because the service is in demand, such as OWO, Kissing etc. Like mentioned by other punters this sort of thing is nothing really special and many will do it in their civvy life anyway, but if they could make more money by structuring their price with extra, why not??
2. They do not like the service. This things could vary from CIF, CIM, Anal, etc. to the more extreme things such as hard sport, fisting, snowballing, etc. Few might like this service such as a porn star but this service is not a norm for a normal escort. It could also be a minor thing that they do not like to be offered to other people apart from her OH. This might include kissing, OWO (to some extend) etc. ….
So I fully believe it is nothing to do with minimum price structure. So even they already charge £150+ if they thing they will make more money by structuring their price with “basic+extra” why not ???
You might argue £150+/hr should include any extra, which I fuly share considering the avarage hourly rate of WG here in the UK, but by the end of the day they will decide the price they charge and the punter will decide wether to take it or to leave it ...