As of 1 January 2000 Four Star leaded fuel has been removed from general sale in the UK. It remains available at a few petrol stations. See the link at the bottom of this page to find one local to you.
Lead in fuel provides two benefits; it coats the valves and valve seats and protects them from wear and boosts the octane rating. Valve seat wear is predominantly a problem with cast iron cylinder heads where the valve seat are cut directly into the head. The valves & seats fitted to Commando cylinder heads are considered to be suitable for use with unleaded fuel.
LRP is no longer available in the UK. Super Unleaded is available but it doesn't deliver the same octane as the old 4 *. Pep it up with a lead substitute/octane booster from your local motor factor.
At 97 octane LRP, delivers the same performance as 4 star and most Commandos can run on it without modification. Combats and tuned engines may encounter occasional "pinking" and may need to be run on Super Unleaded dosed with up to 50% Avgas (note : not LRP, see later).
In summary therefore, LRP is considered to be a very satisfactory alternative to 4 star for most Commandos. It is generally available at roughly the same price as the old 4 star and requires no modification to the engine or ignition timing.
LRP uses sodium in place of lead to protect the valves & seats. However the concentration is considered to be too low to provide effective protection. Therefore LRP is not recommended for sustained high speeds, e.g. motorway driving, in those engines with "soft" valve seats (i.e. not Commandos). Further, the sodium based additive is not "compatible" with tetraethyl lead so alternating between LRP & 4 star or lead additive is likely do more harm than good. Likewise LRP should not be dosed up with Avgas or lead additive; use Super Unleaded as the base fuel if you intend to do this.
4 * leaded petrol continues to be available from a limited number of filling stations around the UK. Visit Bayford Thrust for an up to date list.