Didn't see it myself but my mate in Coventry sent me a video last Friday morning saying something similar happened near him the same night. Different location obviously but the triangle formation you're describing matches what he caught on his phone camera. His footage is a bit shaky but you can clearly make out three distinct points of light holding a steady formation before they just... stopped moving altogether for a good few minutes.
Was the movement on yours completely silent? That's the thing that gets me with these triangle reports, no sound is almost always mentioned. Also how high up would you estimate they were? My mate reckoned his were low enough to rule out satellites but high enough that he couldn't make out any solid shape connecting the lights.
Has anyone cross referenced this with the NUFORC database yet for that date? Worth logging it on there if not already done.