Anyone in North Birmingham can find Birch Ply at Great Barr Sawmills, on the Aldridge Road, just Behind ASDA.
They also stock Sundeala board which isn't stocked everywhere, and if you buy a full sheets of any sheet goods, they also provide a free sawing service. The other day, I even managed to get them to saw a large off-cut into car-sized pieces.
(The walking stick works wonders!)
I have nothing whatever to do with GBS, other than as a fairly regular customer. Some of their timber is OK, but the hardwood is in limited quantities and is priced above usual levels.