FSA is just a more strict body for financial services companies...
exxness is only registered with some local bodies like one from newzealand and one from cyprus ( wtf? ). Although its got an ISO certification as well...
My best guess is that these are easier to register with and have lesser requirements. So there could be a chance of mismanagement/fraud in such companies. Since london is one of the largest financial centers of the world, it has much more strict code of conduct so there is lesser chance of ghappla there. And also its generally better to stick to those who are older in the market, say 10 years old etc..
And when everyone else is offering at max 200-400 leverage and someone offers too good to be true 2000, some thing could be wrong. I hope its not and you get your money.
Since I dont trade, I dont know from personal experience which one is good and which one is bad. ask mohtashimiqbal. he would know.