As said, it might not be mites, so best to see a vet.
However, the vet does just prescribe xeno 450 spot on, which you can buy online at vet uk. You need to apply this every two weeks for three weeks....to all buns. You also need to clean out your room, bedding and cage area well. I sprayed my room with johnsons mites spray, i got this in Pets at Home.