I had tonsillitus on and off for a year once every two months each bout lasting two weeks until I got mine removed this year so I got very used to the whole thing!
Basically, the penicillin does normally help after a few days, but I've found it can get a bit worse in that time so it doesnt sound too unusual. If it keeps up then you can just call/pop in and get them to change the prescription or give you a longer one. I got a second dose quite a few times.
What does your throat look like? Is it very inflamed and spotty? Is one side very much larger than the other - as if this is the case you need to see your GP asap.
The best thing to do is to keep alternating between paracetamol and ibruprofen as much as you can, as it should help you feeling so rubbish. Use a cold flannel etc to help the fever. If you are vomiting then try to have some water/dry toast if you can rather than full on food but keep eating even if it hurts cos otherwise you will feel much weaker.