Most gay male escorts work independently. So yes, lots use Gaydar's escort/client chatroom (but be aware that Gaydar care about the money much more than the safety of people paying for escort memberships). 'Other online ad sites exist' including A-M-E who have never really recovered from losing their high Google ranking.
If you're in London, you should at least consider the scene weekly free magazines Boyz (who got sniffy about explicit ads earlier this year) and QX (who aren't).
Independent work can be as safe as casual pickups, either on the scene or online. Whether it's a commercial arrangement or not, there are lots of things that can be done to make it safer, from the absolutely basic (like letting someone know where you are and when you expect to be finished - make sure the trick knows you'd be missed) to the more involved (getting a landline number and checking it's linked to the address)... and lots of men don't bother.
If you have someone as a 'silent alarm' - if you don't call them by a certain time, they call you and if you don't give them a codeword, they call the police - I think you'll be doing better than most gay male escorts in my experience. The other big safety issue to think about is drugs: given their prevalence on the scene, you need to think about what you're happy with and even if you would do drugs with a client, think very carefully before taking anything they're not taking.
If you're determined to work for an agency, look at the gay monthly press for some ads from agencies.