Business-class seat or first-class seat on SQ?

Business is good enough for me, i'd find first overly-attentive LOL

unless suites are available, go for suites

You have to choose your plane well if you are going for first class. if you can save up that amount of miles, it is worth it for an once a lifetime experience. after that, you will go back to business class.

How do you feel about business class on SQ? If that's good enough for you, then there's no point in spending more for first class. I would however, splurge on suites.

The more important question should be whether you're able to find award availability for SQ first class/ suites from Singapore to London and whether you have enough time from now till when you have to book your trip to accumulate the 100k miles you lack.

Good luck with your search.

Redeeming First class is for a once in a lifetime experience

Business is the more practical option especially for long haul flights since the businesss miles redemption value is already quite high. Just stick to redeeming businesss if you have already taken SIA first class seats before.