My vet said heartworm "doesn't exist" in the UK. My boy's on Advocate for flea control, which happens to do heartworm control as well, but my vet said you couldn't get the heartworm tablets and it wasn't a concern here.
That's not strictly true in the sense that there have been cases of heartworm in dogs reported in the UK - usually associated with travel to the continent where in many countries heartworm disease caused by dirofilaria immitis (the parasite that is common in many parts of the US) is endemic.
More important is protection against a parasite called 'lungworm' also known as the 'French heartworm' (angiostrongylus vasorum) which is native to the UK (mostly the south) - not to be confused with crenosoma vulpis a different type of parasite also known as 'lungworm' but not yet as important in the UK - phew!
Advocate will cover your bases as it will kill not only heartworms (dirofilaria) if your pet is traveling but also lungworms of two varieties one of which is important as it is native to the UK, as well as fleas/ear mites/lice/sarcoptic mange and demodectic mange - not to mention some intestinal parasites in the bargain (hookworms, roundworms, whipworms!). :-)
I don't work for Bayer - by the way, I just talk about these things...a lot. I am equally a fan of Stronghold! Frontline is OK, it only kills fleas/ticks/lice so I'm less excited about it although it has it's place.