I've got similar symptoms (and similar car) with power lose, but only once a month or so. For me, driving with my own leg (not with cruise control I mean ) the car feels to shut down or something, drive just stops for a second and then comes back and drives normally. No codes after that showing. My bet is that there is something wrong with wiring's or connectors between throttle body and pedal.
For high idle, again, I had same symptoms. It was caused by cracked exhaust manifold and leaking pipe joint after 2nd lambda. Fixing those fixed hanging revs, especially when cold engine.
When I last had the MOT done in March there was an advisory. Rear Exhaust has a minor leak of exhaust gases (7.1.2). If this got worse could it be causing the high revs?
Don't know, but i quess that if only the rear exhaust has a leak, it should not mess up the idle. Mine was just near 2nd lambda and it really made idle messy.
I bet that if you take off the exhaust manifold cover there is a big crack on manifold (if still original). That wont show on MOT, or at least not at my case, but will cause revs to hang.