I left my UK passport in my trouser pocket in Cornwall on a recent trip.I completed the trip on my Australian passport,and there was no issue whatsoever.I did not have a UK visa so the immigration officer asked me to sit on the "naughty boys" seat whilst her superior took my Aussie passport to the back office.He returned 5 minutes later with photo copies of my UK passport and admitted me.
It was very calm and pleasant,and the official remarked it happens all the time.No need to worry.