Given suitable gearing (which it sounds like the OP has), Bromptons climb hills just fine; it's everything else they're varying degrees of awful at. Fitness can make up for their inefficiency on the flat, but there's only so much off-roading you can do on one before the comedy runs out. On that basis, I'd treat NCN routes with caution. An OS map would be a better planning tool.
The convenience of a folding bike for multi-modal journeys and for secure storage can't be denied, but unless you're setting out to prove something or have no other option, a Brompton would be very low down my list of choices for a 900km tour.