There was an item on the radio last night about why immigration from the EU in general has dropped massively. Top reason given was the drop in take-home pay due to the drop in value of the pound, second reason given was the UK is perceived as a less attractive place to work, e,g. difficulty getting back into the UK post-Brexit. Poor feedback from those already here means other places are (rightly or wrongly) seen as more lucrative and welcoming options by those at home deciding where to go. That was for immigration in general, but no doubt applies to escorting too.
However, Brexit can't be the only reason. It doesn't explain the drop in number and quality of Asian girls, for example.
If a girl is no good, trade will soon dry up and she will give up and go back home. The girls that are good at their job, who earn enough to make it worthwhile staying here, sooner or later start jacking their prices up. So for the cheaper-end of the market we are reliant on a constant supply of new girls.
Soon after we joined the EU there was a large influx of Czech and Baltic girls, which (along with the internet) totally revolutionised the punting scene. However, as their economies picked up, so there was less incentive for girls from those countries to come here to earn. Just as that tide waned, Poland joined the EU and there was an influx of Polish girls (some outstanding girls at real vfm - that was probably my punting high-time). Then along came the Romanians (really cheap, but unfortunately also really poor service). However, the Romanians also flooded the market, there were so many of them, and so cheap. Years back, most agencies had on average about a dozen or so girls on their books, some (such as admiral) more, some less. Nowadays the number of girls on the agency websites is staggering. However, the competition meant others earnt less. The number of Asians coming here, for example, dropped considerably. Now the Romanian tide is starting to wane, there seems to be an increasing number of South Americans, who have always charged more (higher expenses to cover?).
Unless there is a new influx of girls from a poor region somewhere, I doubt things will improve. Unfortunately I can't see that happening while we make it harder for people to enter the UK, and while the value of the pound remains so low.