You can get generic Heartguard at a cheap price, no RX needed. The price already includes shipping. I have ordered some human drugs from the place for years, they are reliable. www.inhousedrugstore.com
I have read an article that Revolution has not been tested in the United States for ticks other than the dog tick so they cant label it to kill other kinds when I know people that use it say it keeps all ticks off deer, dog and any other kind we have around here. I use Frontline for external parasites and ivomec for internal and it works for me, doesnt mean it will work for you.
That U.K. site is really expensive compared to say, PetEdge. My vet gives a 20% discount for idiots who have three or more animals, so his price is comparable to PetEdge. Plus he checks my dogs for hearworm every year. If you give heartworm meds to a dog that is heartworm positive, you could kill your dog.
I think it's best to get the meds from your vet. If the vet charges more than list price for the meds, get a different vet.