Things with panniers, white helmets with police on back, 1 silver, 1 white and 2 black I think Sorry for late reply been at the beach
Saw some on new VFR1200s last week refuelling in Bagshot. Extremely small Police markings on their backs as you said with white helmets
They could have been students. Police bike courses usually run 3 students + 1 instructor and use plain bikes, and the instructor could well have been Phil Bingham, the first UK Police rider to be given an operational BMW in the 70's but who is now a civillian instructor with Hants having retired from Thames Valley.
Seen them along the Embankment by Battersea bridge, they are the diplomatic protection team. http:// https://farm4.staticflickr.com/3784/11273962186_9f641fdd84.jpg
Seen them along the Embankment by Battersea bridge, they are the diplomatic protection team/special escort group
It is by all accounts, have a few mates who are traffic pol and they said it's good when you have to escort high value person's from a to b but boring sh$@less when it's a slow move.