Make sure you get GRP not plastic (handles break off) and make sure you get ones that are type-approved for marine use for below-waterline thru-hull fitting or else your insurance may not pay out if they fail and your boat sinks!
Here in the US the brand name and material name is "Marelon". These seacocks and thru-hulls are made of a black colored material that is extremely strong. Mr Hawkins was definitely correct when he said to make certain you obrtained thruhulls and valves made for and intended for below the waterline use since you certainly don't want an insurance problem or a sunken boat.
personally mick I wouldnt touch a plastic stopcock from the local plumbers or builders merchant, the chromed brass ball valves are ok but gate valves and plastic valves.. NO THANKS