12.5m is no problem. Small sailing ships can get through the Caledonian Canal. I don't have the latest details here (try www.scottishcanals.co.uk) but an old pilot book lists the air draft as 36m / 120ft. The low point is now is the new Kessock Bridge at Inverness. Apparently two small Sail Training Ships came all the way through - then wnet all the way back as they didn't fit the bridge. But most yachts won't have any problem as this is very high by normal standards.
Have fun, certainly worth the trip, but bring big fenders for large locks full of hire motor cruisers
A pair of Norwegian sail training ships went through the canal successfully but couldn't pass under the kessock bridge an had to go back again. Clearance under the bridge is 23 m. I went under with an air draft of 18 m which looked a lot tighter than it really was!
No problems with your mast.(unless you follow a motor cruiser under an unopened road bridge as many hire yachts have done) I am about 12 miles from Loch Ness and boats are now going up and down the canal. There is no F&M here but the road trafic delays at the dip on the A9 were long at easter.
