I'm sure you know, obviously, not to use that as a way to find out, where I donate blood they make you sign a form saying that isn't why you are doing it, anywho....I'm pretty sure it's damn accurate, I would assume that they hold the blood for a certain amount of time as well before they test it.
Do not use the blood collections service as an HIV testing centre. If you are registered and have had a risk it is better you take yourself off the register for the pre-requisite time.
You should NEVER use a blood donation point as an HIV test. There are special places to test for HIV, like sexual health clinics.
To answer your question, though. Yes, they do test any blood that is donated for HIV. I do not know the method though. I would assume that it is as accurate as general HIV tests.