The generic GA white is frequently called "Matterhorn White" . I have found that Dupli-Color BGM0387 / WA5111 is an excellent match for minor touch ups on many generic-white aircraft. It can be wet sanded and buffed to blend with the existing paint. I don't know if there is an equivalent brand on your side of the pond, maybe they call it Dupli-Colour over there? For a more durable option or larger areas, you can now buy 2-part catalyst cured polyurethane paint in a spray can. You get 24 hours to use it once you push on the big red button to activate it. However, it generally cannot be cut and buffed as all the gloss is in the outer skin.
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