Smart British girls will be doing what most cash-based businesses do: register with HMRC, declaring enough tax to explain their provable purchases (house, car, holidays where your passport gets stamped) and general lifestyle, then using undeclared income for spending on fripperies. The general rule is "don't take the piss". Hector's not going to open up a full tax investigation for 5 grand spent on shoes, clothes and fancy meals, as it will cost more than that to find it. Buying a house or car with undeclared income is painting a big red target on your back though.
I'd imagine most of the EEs are shuttling it out of the country in their luggage.