I can't give you any kind of scientific comparison, but with really good preparation I get about 5 years out of exterior gloss - using either Dulux exterior or the stuff I get from the trade shop. I have 20+ sash windows and other assorted exterior wood.
You can get marine paints from any chandlers which are more often located at either canal or river locations. Try google for your local area.
Screwfix also do a limited range of colours, but I have no idea how good they are compared to the more expensive makes. Their colour range is also quite limited.
If your not bothered about a brightly coloured finish, products such as cuprinol or the oil suggested by masona are easier to recoat in the future because they need much less prep.
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