Cost massively
But also they are more willing to take on things like high bmi, amh scores etc. Lots of the lower cost clinics in the uk have very strict criteria, but even when you're paying the full whack then they are still selective.
They also have much higher success rates. For us they had more interest in trying different things. For us we only have the money for a select number of rounds so the poor success rates of uk weighed heavily
Lots of the people I know that went abroad went for egg donation which has lengthy lists in the uk and not much choice
We had some bad experiences with uk clinics losing paperwork, missing appts and generally poor care. They were very "this is the protocol" sorry if it doesn't fit but we've started it so we will finish which is tricky when its tens of thousands of pounds
The big few for tourism were cyprus, Spain, Portugal, Czech republic.
North cyprus is attractive to lesbian couples because theres a surprising amount of countries eg Germany, Italy, Czech republic don't allow same sex couples. Lots of same sex couples/single females/ people who don't meet their countries criteria seek outwards
In general people look for transport links, one who allow you to use uk things for scans, and affordability.
For me we were weighing up waiting to meet the criteria for affordable ivf and the impact that has on fertility, or going abroad, and paying more for poorer care and less likely for success