She could probably have charged per hour and collected at least £80 assuming only extras were DFK and OWO. I can see a scenario or explanation where most reasonable WGs charge between £0 and £80 for those two extras for that time. Boudoir's reputation has for years been you could easily have been charged 2x or 3x that. Either by charging £20 for OWO, charging per half hour or them just making it up on the spot. However, I believe they do have an extras price menu up in the rooms now so perhaps they are trying to improve.
Consider that the £400 was probably divided £160 to the parlour £240 to the WG and if this was her only booking of the night was likely £190 parlour £210 WG.