Prepare for painting with sugar soap or fine sandpaper?

Even a five minute rub down with fine sandpaper makes a big difference. Use as much undercoat as you need to hide the imperfections, then one thin coat of gloss

I've always had good results using sugar soap and giving the paintwork a good scrub with the rough side of one of those pan scouring sponges. Then a final wipe over with white spirit. The only mistake I've made is thinking I could get away without the undercoat, looks great when done but starts yellowing in a matter of weeks

I was also thinking along these lines having a similar job to do. So I've decided to try some of that wet 'n' dry sandpaper using a sanding block and a bucket of sugar soap solution in one go, seems to work OK but I also wiped down with some old rag immediately, this seems to help get rid of the dirt. It takes a while though, I don;t think it'll be safe using an electric sander with sugar soap. :)

That's a false dilemma. You use both, if you care.

If forced (especially anywhere near a kitchen), I'd go with the sugar soap alone. Unless you've already washed down and degreased the paint, sandpaper used alone will clog before it achieves much. I wouldn't even use sandpaper - this is definitely a job for an open mesh abrasive, such as wire wool or a plastic mesh Webrax/3M if it isn't too bad.