The size of the Thread is 1/2" BSP, The size of the pipe is 15mm. These can be cut with a standard 15mm pipe cutter. Microbore Copper Pipe -Tube - 4, 5, 6, 8, 10mm Gas Water Various Lengths 1m-10m. Used to extend the reach of a radiator valve.
3/4" x 1/2" Union Radiator Valve Tail, Chrome, BSP Threads
3/4" x 1/2" Union Radiator Valve Tail (Single item)
Radiator Valve Tail 1/2 Inch Replacement Connector Brass Fitting
2x 15mm Radiator Valve Pipes and Collars Easy Fit Packs Plumbing Tubes Brass
Drayton 15mm radiator valve