There are nominally 2 sizes of 1/2" tap valves (and hence washers) available, BS 5412 or BS 1010, one is a coarse thread on the valve the other is a finer thread, washers differ slightly. Both size washers are easily available as are the odder similar nominal sizes.
Note that tap reviver kits are available, again in both threads (and a universal set with adapters) and in cross head, BES will have better quality than screwfix I think, as mentioned you might need to reface the seating if the seat has eroded.