Defining what stations count as 'big' is required here. A few that come to mind:
London Victoria, Waterloo, Paddington, Liverpool Street, Bridge (east end of p5)
Leeds
York
Newcastle
Edinburgh Waverley
Glasgow Central
Cardiff Central (east end of p3/4)
Bristol Temple Meads (north end of p5)
Brighton
Ashford International
Manchester Victoria, Piccadilly (end of p8 )
By no means an exhaustive list of 'big' stations, but hey.
As for the above, I've put in brackets the parts of the station recommended for spotting/photography (if known). Those that aren't detailed I haven't a clue so can't recommend any spots.
But I know what you mean klambert, it's exhausting racing around places like Newcastle to get everything! London Bridge is easier for the best spot but is just a massive strain on the wrist when frantically writing everything down in the peaks.
No magical way to manage a long spotting session at large stations, although before starting a session at Bridge a big coffee and something substantial to eat would be recommended to keep you going! You just get used to rushing about the big stations eventually though.