There are two main types of HIV test available in the UK today: 4th generation and 3rd generation HIV tests. (The 1st and 2nd generation tests have now been superseded by more accurate tests).
The Better2Know laboratory HIV test is a 4th generation test. The 4th generation test tests for both the antibodies of HIV type I and type II as well as the p24 antigen. 3rd generation HIV tests only test for the antibodies (type I and II).
The main difference is that the 3rd generation test has a longer window or incubation period in between testing is 90 days, but a 4th generation test, because the p24 antigens appear first has a 28 day incubation period. This is the minimum number of days you need to wait between the incident you are concerned about and getting your HIV test. 3rd generation tests are still being used by some NHS clinics and other primary care providers.