BS4504 PN10 Slip On Flange is your good choice."BS4504 Pipes, Valves and Pipe Fittings with Round Flanges" This is only a standard, which is a British national standard.
BS is like my country's GB, which is the British Standard. The following number 4504 is a serial number just like 150 in our country's GB150, which is meaningless. BS4504 PN10 Slip On Flange is your good choice.