Some have been covered already, but mine would be:
Pad to Pembroke Dock (summer only)
Eus to Holyhead
Kgx to Aberdeen (tight in a day)
Waterloo to Weymouth
Pad to Penzance
Sleepers to Fort William and Inverness
A bit left field, summer only Liverpool Street to Great Yarmouth
Pad to Newquay, though last bit to Newquay is dull in my view.