Where do you get your medications compounded? Do you get heartworm meds compounded?

I read somewhere online that it is possible to get heartworm medications compounded at a veterinary pharmacy but I cannot find a pharmacy that will compound heartworm medicine. Where do you get your dog's medications compounded? Do you get their heartworm medicine compounded? Thank you

(Compounding is where a pharmacy makes a medication specific to the dog's weight and needs.)

We are UK
I'm not even sure we even have this ? Think vets just it prescribe ready made doses -Sizes pills etc