Marine Spar varnish formulations are typically a mix of oil, resin, solvent, dryers and ultra-violet additives. These ingredients keep wood at optimum functionality.
Iím not familiar with what is available in the UK, but a spar varnish that is designed for boats is probably the best you can get. If you prepare the surface properly and apply the varnish in 3-4 coats, thatís the best you can do. Depending on the weather, there may not be a varnish that will last as long as you want. House paints might be better, but look like paint.