Gaz are also a popular option. Depends on budget and if you buy into the addition of the expensive of posh dampers on historic cars with limited/lower motion ration of the dampers to wheel movement.
http://www.burtonpower.com/gaz-coilo...-075r12-2.html
Spax also will make up dampers that fit into the UK rule (steel, single adjustable twin tube) of custom lengths
https://www.spaxperformance.com/racing-home-en/
All these makes can be seen on cars that have won races in the UK series.