In reply to GDes:
Simonside is a brilliant place, and it would be bordering on criminal if the OP were to drive past the county on the way to Scotland and not stop off at some of the best sandstone in the entire country. Simonside is one of the best lumps of sandstone for the low-grade leader anywhere in Northumberland, though I may be a bit biased as my first lead was there.
I've always considered Crag Lough an impressive place for reasons other than the climbing. The outlook is remote and beautiful, and the climbing is different and interesting. Would I drive from St. Just to visit? No, but I'd definitely recommend it to someone who was already driving past.
The fixation on 'the best' climbing is the reason why so many perfectly nice crags are overgrown and perfectly good routes are buried while the relative handful of *** classics are trashed beyond recognition. Climbing is an adventurous sport, and I think Crag Lough is a great place to have an adventure.