Hi Dan and welcome to GADC.
Re Glencoe, I fly there a lot. NTS do not own the whole glen, in fact some of the best images can be had from just outside their boundary. Check out some of my photos here - Rannoch Moor & Glencoe all shot outside NTS land. To be honest, even if you did go right in to the Glen and TOAL, there is not normally anyone going to question you. Just watch out for low flying military aircraft.
Re Glenfinnan, I’ve not been there in a while but I found it easier just to pay the £10 fee. Chatting to Alastair the estate manager, he told me only three folk had paid that day, yet I saw a lot more than three drones in the air.