To keep the polished gel coat white and stop the yellowing you need to seal it with a coating such as a wax. (A few years back we used T Cut to polish the hull - it came up nice and white, but after one sail returned to yellow)
We found the Starbrite 'polymer' coating v. effective. It claims to protect for up to a year and in our experience over the last few years the claim seems to be truthful.
Took your advice and Peter Furmbys' advice and one polisher and eighty quids worth of 3M products later my boat has a finish that dazzles. Well worth the expense and the envy of the club!