She may be a very thorough individual who accounted for all of the following before mentioning them to you, but most people seem not to operate in that way. For starters, she probably takes lots of holidays, but your arithmetic assumes she doesn't. You can almost certainly eliminate 5 earning weeks per year for them; and, given the nature of the work, possibly 10: so that's between -£23k to -£45k of earnings off.
The rest is conjecture and depending on big-hitting factors like her location and the type of service she offers, may have vastly different outcomes, but her expenses: incall location, gear, travel, marketing, personal grooming, do all stack up. Like Dolly Parton said "It costs a lot of money to look this cheap".
Pretty soon, she's deep south of £200k. Bear in mind this business's limited shelf-life, high burn-out rate and that this may represent her peak earnings. I'd expect talented, attractive and diligent city-based WGs to be able to 'earn' £60k pa (£180k over 3 years) - with some earning more, and many earning less. Competent WGs earning around twice the national average doesn't look quite so bizarre when you consider the above, and what they have to do.