I find this all so interesting. Is this all due to Brexit and Poland doing so well / having more opportunities?
I think it's a mix of things. Post 2004 and a lot came to make money.
I can think of a few parts of East London which at the time had a 'sauna' on each high street, each staffed by 4-5 EE (occasionally Spanish and Italian) birds each day. There was lots of regular and predictable safe sex work to go around. As well as that you had static brothels which all advertised in newspapers and shop windows, again plenty pf work to go around for EEs. I recall meeting a LOT of fit Lithuanian girls in their 20s back then.
I was talking to a Polish WG a couple of years ago who was saying it's just not worth it now (and she was quite keenly reviewed, as I recall) as the burdens and overheads of working in a flat in London these days are just too much. Not just the rent but the hassle of sorting ads, losing money through time wasters etc. I think not long after she stopped.
For new entrants it's not as simple as "Start on Monday", which is why the brothels are full of the pipeline that Sergio (not Sergei) can get these days.