It was perfectly still night in the UK. I outstretched my hand and finger and, I forget the details exactly, but the finger just about blocked out the pair of lights I think.
A quick call to my father in law (pilot) and some info on joining the landing pattern at Boscombe Down, added to the lack of red/green lights so not normally a craft expecting to interact with other traffic, and a little investigative intuition seeing as the lights were moving so slow as to almost be gliding - and I find the Zephyr high altitude long endurance surveillance aircraft.
https://www.gov.uk/government/news/mod-buys-third-record-bre...
A bit of trigonometry showed with a wingspan of 35m (I think that was it) and the angle of my finger outstretched, put the craft at about the height it should be for following the standard approach path into Boscombe Down.
So with a little knowledge it was possibly to determine with a high degree of likelihood what this 'UFO' was.
I suspect that virtually all reports can be attributed to similar terrestrial origins with experimental aircraft and using the 'extra-terrestrial' card is just a convenient way of covering that up.
EDIT - I should add I saw it because it was I think the night SpaceX launched and landed a reusable rocket maybe for first time? Or perhaps it was reused rocket with reused dragon cargo capsule to ISS. The path I think was going to take it over the UK anyway, so I was out to see if it would be visible, having watched the launch a little earlier in the evening.
Note to Royal Air Force - don't land the slightly secret plane when many people is likely to be looking up!