Monistat is miconozole (antifungal) and the UK version (Daktarin) is not available over the counter in the UK for that particular area of the body (as far as I know - at least the Daktarin cream behind our counter in Boots is only for sweat rash, ringworm, nappy rash etc. and not for thrush).
You could try Canesten 2% Clotrimazole (external cream) or a Canesten Pessary, Canesten Once (cream in a pre-filled applicator), or Oral Tablet (Diflucan/Canesten Oral Tablet/Boots own tablet), but if you have already tried any of these and they are not working, you will need to go to the doctor/GUM clinic.
In order to sell Canesten over the counter, healthcare advisors have to make sure that you haven't suffered with it more than once over the past 6 months before it can be sold to you, so if it is an on-off problem and you have suffered with it twice or more in the last 6 months, you really need to go straight to the clinic/GP to check if it is being caused by an underlying condition.