Unipower (or more accurately Universal Power Drives Ltd,) did sell Pole Trailers.
I had always thought they were badge engineered, though others claim Unipowers product range to include Front wheel drive, rear engined cars, Aircraft Refuelling bowsers , Aircraft tucks, four wheel steer dock shunters, pen cast mining shovels, Gun tractors, Logging arches, Timber drugs, third axl;
e convertions and oilfield tractors.
Unipower are famous for extra axle conversions to trucks, so would have a ready supply of Unipower badged hub caps.
The Woods Brothers at Rayleigh, have one they tow behind one of their Unipowers, as far as I know.
Wynn's ran three Unipower Timber tractors at least. The Caps could therefore have come off one of these, if they scrapped any of them.....