I've often booked then moved seats. Guards don't seem to mind at all, and this is on a few TOCs, they are only bothered you are on the right train. One thing I always do though is take the reservation slip out (never had one where there was another reservation after me, but I'd either leave it in or put it back before I got off), so that someone looking for an empty seat knows it's no longer reserved. Technically it is against the rules, but in reality it's the fairer option. Of cource, on trains with electronic reservations that can't be done.
Last time I moved seats was on an MML Meridien from St Pancras. I'd gone down in the morning on a busy peak hour train (it was the semi-fast that runs via the Wymington Deviation, the part where the slow lines divert around and under Sharnbrook Summit, so the Master Cutler can pass, which was one of the few bits of the MML I needed), so had booked to get a seat and to get a cheaper price. Coming back though later that evening I was reserved in coach A seat 1A, an airline seat at the back of the front coach, right next to a window pillar. The only other seat that seemed to be booked on the whole train was seat 2A, the aisle seat next to me. Naturally I moved to a table to spread out, and infact other than the driver, and briefly the guard when the came to check my ticket, I was the only person in the carriage for the whole journey.
ign5: . Some of them should try commuting into Glasgow. On some trains, a seat is a luxury!
