Until recently, Phoenix was seen as the leader in the UK trg market, however it's since been bought over by Armor group and it's main focus has switched from producing effective operators, to making as much cash as possible. If you choose to go on it, expect large student classes with insufficient instructors.
Task used to be one of the more decent ones, it certainly isnt now. I recently did the course, instruction is aimed completely towards the London scene. Its more of a text book regurgitation course designed to get you through the SIA required exams. On the plus side the standard of instruction, student instructor ratio and accomodation are quite good.
Phoenix, heard mixed reports mostly on the plus side, however the instructor to student ratio is not good. With Phoenix now being owned by AG, you are pretty much guaranteed an interview with said company at the finish. Saying that they pretty much know who they are offering a job to with course reports etc. Basically if your a mong you will get the interview but have already been black listed due to your report.
Ronin South Africa, have heard some excellent reports on this company. 6 weeks in south africa, which includes a good range package and excellent medical skills week.
Nemisis, cant comment on this lot.
Your best sticking to one of the larger training providers however. A good pass from Phoenix/Ronin will mean a lot more to a potential employer than one of the smaller less well known training providers.
Ronin having issues with HSE First Aid At Work as its deemed they cannot issue a certificate as they train outside the UK (same problem with HSE for guys serving in Cyprus btw).. Which is frankly ridiculous as they arguably offer a unique and quality practical medical package. Could mean you have to get first aid certified else where, but you cant discount the actual hands on medical training they deliver.