We go here with the dcs, they also do a posh dinner in a posh carriage thing that I haven't tried, but it's nice, not too long a journey and if you let them know you're interested they're very keen to let you go into the engine etc. www.bluebell-railway.co.uk/
Steam Trains these days rarely go on Main Line... but Steam Dreams does arrange such trips. One of the locos used is 60019 Bittern which was recently restored at Mid Hants Railway. As a Life Member of that railway... felt I should highlight one of our Engines being used for Main Line runs. Steam Dreams are doing a number or runs with Bittern in December 2009, from London to various Cathedral Cities.