"To make your Christmas ‘ham’ you’ll need to buy a gammon and cook according to your recipe and preference."
Both gammon and ham are cuts from the hind legs of a pig. Gammon is sold raw and ham is sold ready-to-eat, Gammon has been cured in the same way as bacon whereas ham has been dry-cured or cooked. Once you've cooked your gammon, it is then called ham.
"To make your Christmas ‘ham’ you’ll need to buy a gammon and cook according to your recipe and preference."