Not all British built Raleighs use 26tpi. My Royal according to internet sources was built in September 1983 by the Lightweight Unit at Nottingham. This unit was the successor to the Carlton Worksop factory. These bikes used BSC threads so standard component fit without any problems. If the serial number starts with a "W" it is likely that the bottom bracket is a standard 24tpi one. The W means that the bike was built at Worksop or the Lightweight Unit. This unit had the skills and equipment to build frames with 531 tubing.
Not sure if I knew this when I put in a new bottom bracket about 10 years ago. However as this frame approaches it's 40th birthday I still enjoy riding it. The frame and forks are all that are left of the bike I rode home from the shop in 1983. I remember how it felt, the bike was so much more responsive than the Peugeot framed bike I had before.
I swapped components from a smaller bike to the Peugeot and everything fitted, I now realise how lucky I was.
531 is pretty run of the mill and didn't really require special skills, I think it was just convenient for them to carry on there when they bought Carlton. The SBDU at Ilkeston made all the special sauce, latterly wrapped up and moved back to Nottingham and renamed the Special Products Division.