Not easy to photograph...

The following all qualify as British mammals but regrettably getting a photo of them in the wild is unlikely since they became extinct in this country some time ago. (Wild Boar and Reindeer have been successfully re-introduced and groups are seeking permission to re-introduce Wolf and European Beaver.)
Spotted Hyena – c32,200 BC
Bison – c25,650 BC
Woolly Rhinoceros – c22,350 BC
Wolverine – c20,160 BC
Woolly Mammoth – c10,800 BC
Arctic Fox – c10,000 BC
Giant Irish Elk – c8,960 BC
Reindeer – c6,000 BC – subsequently re-introduced
Aurochs – c1,000 BC
Brown Bear – disputed departure dates ranging from 1st to 8th Centuries AD
Lynx – 180 AD
Wild Boar – 13th Century but re-introduced a few times since
European Beaver – allegedly c1300 but may be earlier by a few hundred years
Wolf – c1690 AD
St Kilda House Mouse - c1930

The Reindeer and Wild Boar are the only two now roaming wild in this country and as the Reindeer herds are fed only the Wild Boar can be said to have truly re-established itself. They have even set up their own website!!!
http://www.britishwildboar.org.uk/