The negative test that you had at 8 weeks is certainly highly reassuring and a very good indication as it is possible to detect the HIV antibodies already at that stage. However current
UK guidelines still recommend to have a final
test at 12 weeks for it to be considered final and fully conclusive. Having said all that, and speaking out of my own clinical experience, I have never come across anyone testing negative at 8 weeks and then positive at 12 weeks.