It's not a BSP thread, it's a special size thread for Siamp flush valves. Genuine Siamp part identified by unusual design on nut face and outline of flats.
eBay
For Siamp Optima 50 wired drain valve. Simple installation and maintenance. 1 x Drain valve button（with retaining cap）. This item have 3 months warranty. Because, our prices are unable to include above fees, as the custom policies are different in every territories.
eBay
SAVE MONEY AND SAVE WATER WITH THIS COST EFFECTIVE DIY SIMPLE FIX 1 X SIAMP 33a / 45 STORM SKIPPER GENUINE SIAMP SPARE AUTHENTIC PART WATER SAVING REPLACEMENT DROP VALVE WASHER FLUSH VALVE SEAL EASY DIY FIT EXPRESS POSTAGE OPTION GLOBEL SHIPPING OPTION
eBay
1/5 Drain valve sealing for ring. Compre ssion Connection: Designed with compression inlet and outlet connection types, this washer ensures a secure and leak-free fit, providing peace of mind during use.
eBay
Genuine Siamp Spare. Replacing this for other Siamp valves allows you to install a new one without having to unbolt the pan and cistern making it so much quicker and easier! The Optima S flush valve will replace an Optima 49, 50, Skipper 45 & Storm 33a also.
eBay
Siamp Compact 95L WC Toilet Cistern Side Entry Inlet Valve. Compact design with integrated float system. Not sure which size you have?.
eBay
【Single Flush Toilet Push Button】: Single push flushing toilet button, can saving water effectively. 【Easy Installation】: This toilet water tank button is easy to install and use. The modern color smooth finish adds a modern touch to your bathroom decor.
eBay
1/5 Drain valve sealing for ring. Compre ssion Connection: Designed with compression inlet and outlet connection types, this washer ensures a secure and leak-free fit, providing peace of mind during use.
eBay