This one's at the budgeting stage right now...

Next year, I would like to build my own workshop from scratch - simple
timber construction, insulated and very very strong. I need a weird shape
to avoid planning issues (the 5m rule from house rule - I need a chopped
off 45 degree corner on one bit) so standard offerings are unlikely to be
much good. And it will be fun to build something structural from scratch

Any tips on a good source of wood that's solid, stable and inexpensive?

(I reckon I would need a few 6x2's for the floor beams, 4x2" or 3x3 for roof
and walls and something to clad the outside).

I was thinking decking timber for the structural bits and the floors as it's
fairly ubiquitous. Is there a better source and how much might I expect to
pay per metre for say 4x2" ish (so I know what to shoot for).

The whole thing will sit on brick or concrete bearer walls so I can store
long things (ladders) underneath and get good airflow. I'll be insulating
it and lining the inside walls too.

Ta

Tim