Combining the narrow gauge lines with national rail network you can get round most of Snowdonia. Agreed it could be a bit of a hassle moving kit and bikes.
Best lines in the Peak to ride from:
Sheffield Manchester line through the Hope Valley or Buxton to Manchester line. Macclesfield station gives you good access to the Macc Forest / SW Peak.
Rugeley / Cannock stations for Cannock Chase
Church Stretton for the Long Myndd
Craven Arms to Llanelli Line for quite a lot of mid wales riding.
Settle Carlisle Line for Yorkshire Dales
Aire Valley Line for S. Pennines and Dales
Calderdale Line for S. Pennines
Lot of the new v-graphics guide book rides for South Pennines seem to have station starts.
Esk Valley Line (to Whitby) for North Yorkshire Moors. Link with North Yorkshire Moors Railway to get to Dalby etc.