I'd like (with my fantasy hat firmly on):
Clacton-on-Sea to Reading using Stadler 745s. For no other reason than I would have 12 coaches to myself!
Lincoln to Whitby, calling at Gainsborough Lea Road, Doncaster, York, Middlesbrough, Battersby and Grosmont, using 185s.
Cottingham to Mottingham via Nottingham, using Pacers in 10-coach rakes.
Westminster tube station to Scunthorpe, using convertible 80x units. Routed via Aldgate, Amersham, Aylesbury, Bletchley, and Manchester Airport. The 80x units must not be given Pacers bench seats as that would be an upgrade!