Short version - I went to a shop.
Middling version - Venison is quite widely available in England. The more upmarket supermarkets tend to stock it, so getting hard of it is not difficult. Farm shops and farmers markets are also a commonplace thing in England and venison is readily available at those places.
Long version - on our drive back from Scotland, we passed a deer farm with a farm shop. My wife, who lived up in that part of Scotland, confessed having been past the farm many times over the years but, had never been to the shop. So we stopped there to get a bite to eat and I perused their selection of meats in the shop (which wasn't limited to venison), before selecting a nice haunch to take home for Christmas dinner.
"One of the great tragedies of mankind is that morality has been hijacked by religion." - Arthur C Clarke
"'Science doesn't know everything' - Well science knows it doesn't know everything, otherwise it'd stop" - Dara O'Brian