I think your uncle is on a mid 1920s Triumph Model P.
If we take stock of what is in the pic, the engine is a sidevalve (angle of exhaust pipe), the tank is between the frame rails, magneto low mounted in front of engine, the forks are sidespring trapezoidal, mudguards are plain curved c-section, front brake is small internal expanding, primary chain cover appears to be alloy, the rear wheel has a belt drive style of brake band, but no sign of a belt (chain drive was on the other side). And the tank has a Triumph style of paint and lining, although the emblem is not as standout as you would expect.
This all adds up to be a circa 1925 or 1926 Model P. Don't know my Triumphs well enough to be certain of year, maybe someone can confirm this or correct me - or totally shoot this down.. The Model P was introduced as an absolute price cutting model, 42 quid or thereabouts got you a very capable 500cc sidevalve tourer. That alloy primary cover must have cost nearly that to produce however, and electric lighting was a reasonably expensive addition. If it was still in service in WW2, obviously it survived quite well.
http://motorbike-search-engine.co.uk/classic_bikes/triumph-model-p.jpg1926. Couldn't readily find a pic of t'other side, sure to be about.
Opethiselps - great pic.