It is quite possible that you ran into a mountain wave system
I was less experienced when I did that flight and with hindsight can't imagine how we weren't affected further North in Scotland. I now work on a rule of thumb that to avoid wave, I need to be (windspeed x 1000ft) above mountain or hill.
I recall we were about 1500-2000ft agl, winds were circa 30kts so if we'd been a couple thousand feet higher, would probably have been ok.