I recall travelling on a Class 313 from Hitchin to Moorgate in June 1977. It was an excursion train advertised through the local paper, for people going to London for the Queen's Silver Jubilee celebrations, and was also the first electric passenger service from Hitchin - the first 312s between Royston & Kings Cross didn't start till September 1977.
So it's nearly 40 years since that excursion - I think calling it a railtour is stretching things - but now the electrified network is much larger so how could we convince GTR to operate a farewell tour next year, and how far up the ECML could it go?
How about Weymouth to Glasgow via the ECML? 313 special! Maybe the furthest you could do in this country with electric traction (Eurotunnel excluded).