All pet insurance policies available in the UK have a fixed excess amount for vet fees which is deducted from your claim. This can be as low as £55 or as high as £169 with the average being about £75-£80.
Here is a list of providers who offer policies with a fixed excess under £70.
Helpucover (for cats)
Petpals Direct (for cats)
Post Office (for cats)
Surewise (for cats)
Petplan (for cats)
Remember that the excess is part of the cost of a pet insurance policy. You will also want to consider the premium (now and in the future) and the contribution payment if any.
Some policies will offer you the opportunity to increase the excess in return for a lower premium. See Pet Insurance Providers with Voluntary Excesses