Whilst Hippy's information on the Mil traffic is interesting, I'm afraid the real explanation is much more mundane.
Bergi departures head NW from Amsterdam leading into the North Sea sector of London's airspace via ELDIN. The contrails you saw were almost certainly high level CIVIL traffic on the airway as already mentioned. Trying to judge vertical separation of 2000 ft from 24000 ft beneath is a bit tricky!
We would not allow tankers and chicks to operate IN the airway if we could avoid it, so if they are there they will be through MC16 and south of UY70/UL602. Must say I don't often see tankers and chicks en-route, although they do often fly around in the AARAs spread about the London and Scottish FIRs.