Regarding availability of sockets, each coach has it's own separate power supply, so if a circuit breaker in one carriage has tripped out due to a hairdryer being plugged in, then chances are the rest of the train is OK. Even with an advance ticket, I would imagine most TM‡'s would be sympathetic to you changing carriages if seats are available if you explain and ask to move.
The only time I've ever been told off for not sitting in my booked seat was on Grand Central. A friend and I had 2 seats in first on a table for 4, and decided to move to a 2 instead, as it was better for us, and would also mean a 4 was available if 3 or 4 people wanted to sit together. We moved the reserved cards too so no one would think they couldn't sit there! I would have thought this improves things all round, but the guard was really stroppy!
No other guard has cared less where I've been sitting, as long as it's the correct train if an AP.