I recently saw Christina from Bunnies. [...]. Also she's definitely not Japanese as mentioned on the site but she said her parents are which I suspect isn't true based on the brief Japanese I spoke with her (I speak some Japanese) [...]
Given the lack of proper mastery of English of most Japanese and given the fact that escorting prices are way higher in Japan, it just makes no economic sense for any escort to come and offer service in London or in the UK in general.
One happy to accept foreigners would make tremendously more in one of the large cities in Japan (that's also a criteria, many Japanese SPs would simply not accept gaijins) rather than over here.