I have used boots, thin kthey are 25 MUI showed strong line for me. I also tried Sainsburys own think they are 15 MUI (the 4 days early ones) The first one i used was a wilkinsons stores cheapie (£1.99 for 2) at 10dpo and got a very faint line, the yare 25 MUI too but the lines never get as dark as the better tests. Get a variety but stay away from Clear blue digis until you know you are preggers, i used one of them first ICSi and saw a BIG fat NOT PREGNANT at 13dpo and it was devistating, not seeing a line is bad enough but NOT PREGNANT up in lights is horrendous. I used the digi to get an idea of HCG levels at 14dpo cos it has a conception indicator.
Hope this helps