Weald and Downland Open Air Museum.
There's lots along the Welsh border: Leigh Court, Stokesay castle are both great crucks.
The Black and White Trail out of Herefordshire.
Tim Crump's shop (Bill Keir is there).
Ely, Lincoln and Wells Cathedrals.
York is great.
But I would caution that it's not inevitable that someone will guide you up in the various roofs. I've found it's better to contact the works department or chapter house (for cathedrals) ahead of time, say you're a timber engineer, and set up an appointment. You may be asked for a donation, but it's usually no problem to get an appointment and guide if you're a professional builder or architect.
When we went to York Cathedral we were guided by the head mason named Mason who was the third generation working at the cathedral. By the time they finished one circuit around the building it was time to start around again.