For some reason, meat in the UK is either chicken or processed.
Depends on your local supermarket...
I can get Gressingham* duck from my local branches of Sainsburys, and then there is the monthly farmer's market, from which I've been able to procure wild mallard.
Every so often, Sainsburys do a half-price promotion on whole Gressingham ducks - as I usually don't have room in the freezer for a whole one, I'll joint it, and use the carcass for stock.
* A wild mallard/Pekin crossbreed. If memory serves, supermarket-bought duck used to be an Aylesbury/Pekin crossbreed, but Gressingham has more flavour.