Atco Paint at Aldi
I found a source of low cost Atco shade of paint in our Aldi supermarket, labeled as Deco Metal Protection Paint and available in both 400mm aerosol at £2.99 and 1 ltr tin at £4.99. The shade is called Pine Green and the nearest match to Atco that I have seen. There are variations, so ensure it is gloss and Metal Protection range.
Good that you've found an economical source of Atco Green, but it should be noted that over the years Atco used two shades of green (may be more). What might be called the Original Green is a more "muddy" green than the later brighter green used on the Royales etc. Both those shades are available from Central Spares - but not at Aldi prices!
Interestingly, I have some Aldi Deco green paint that is a fair but not exact match to the original Atco colour but it is marked just Green; no other description, Pine or otherwise.