I miss the BTM - OXF runs too. Never had a chance to bash one, but it would have been fun I'll bet!
Other runs I miss include the New Radnor - Kington - Leominster ones, as well as Leominster - Worcester via Bromyard, Hereford - Brecon and onwards to Swansea via Hay-on-Wye and Three Cocks Junction, as well as the route from Three Cocks Junction through Mid Wales, the Heads of the Valleys run (it would be much more fun going to Merthyr Tydfil, Aberdare and the like via this route than the much longer way via the Marches and Cardiff up through Pontypridd and onto the corresponding branch)...
then there's also the link between Severn Beach and Pilning, which would have been great to do!
It'll be noted that all of the above routes and services, bar Severn Beach - Pilning and Bristol - Oxford (not 100% certain with the former), were before I would have been born, these lines being withdrawn decades ago. Indeed, New Radnor - Kington was withdrawn in 1951, Kington - Leominster in 1955. The others followed suit in the 1950s and 1960s unfortunately. If they were still in existance, promoted properly, run with reasonable stock (Hereford to Swansea via Brecon with 158s for instance) and upgraded to run at a speed of more than a handful of miles per hour, then we'd have a much better network. Alas, they've gone, and will never be back.
EDIT: Who could forget the Regional Eurostars and Nightstar? If they were still about I'd be loving it, and I daresay more of the country would be too. If it's one thing people hate, it's the lack of decent connections without staying in London overnight each side of a trip with Eurostar. It's OK for those, mainly, in the NSE area, but outside of that, you've no chance. Rely on the Caledonian Sleeper to ship you down from the Highlands to Euston in time? That's asking for trouble. Those connecting Eurostars now would go a long way to making international travel so much easier...Inter-connecting HSTs from this part of the world (Hereford, Cardiff, Swansea and Carmarthen for examples) would be most welcome too!