Great Britain requires dogs (not cats or ferrets) to be treated for the tapeworm Echinococcus multilocularis before every single entry, because the parasite doesn't naturally occur in GB and this is one of the controls that keeps it that way. The treatment has to contain praziquantel or an equivalent proven effective against that specific tapeworm, and a vet has to administer it and record the product name, manufacturer, and the exact date and time on the pet's documentation.
Timing is strict and narrower than most other pet-travel windows: no less than 24 hours and no more than 120 hours (5 days) before you enter Great Britain. Treat too early and it's as if it wasn't done at all — your dog can be refused entry or placed into quarantine, at your cost, for missing this window.
There's a specific, short exemption list: dogs travelling directly from Finland, Ireland, Northern Ireland, Malta or Norway don't need the treatment, because those countries carry equivalent status for the parasite. The exemption only applies to direct travel from one of those five — if your route touches another country first, the standard 24–120 hour rule applies again.