Hopefully you read this before going in!
I had my tonsils out 4 weeks ago (and am feeling great now!) and the day before i started getting a really itchy throat. They took my temperature before surgery and it was a bit high but they still went ahead with it.
They take infected tonsils out all the time.
Likely they will just give you a strong course of antibiotics to make sure if the infection if anywhere other than the tonsils that it doesn't spread.
They said my tonsils were extremely infected when they took them out and if I wasn't booked in for a tonsillectomy that day I probably would have ended in hospital with a bad case of tonsilitis and had to have them out anyway!!
Good luck with the surgery.