My old vet (Sedgwicks) just sent a bill at the end of the month and you could pay as much or as little as you wanted. As long you payed at least 5% of the bill total each month they where happy. Sadly they where bought out by a big name veterinary group called milcrofts. who want paying as soon as the treatments done. They do have to option to set up a payment plan if necessary, but it has to be done by direct debit.