FTP are not too bad and would echo Luxembourger.
I would suggest that you aim for either something with ScotRail to either Edinburgh or Glasgow, then either East Coast/Cross Country to York or Virgin/Cross Country to Preston. From either York or Preston you will travel on FTP but the journey length will be shorter. If via Preston avoid the Northern journeys as there is no first class. There are some East Coast journeys that start from ABZ.
The man in Seat61 is very good for all things railway, except the UK.
Hope that helps
Edited to mention, avoid Cross Country if you can, colleagues say that it is not that good in first compared to EC or Virgin