I've flown both this sector and also SQ317 from LHR, both of which leave the UK at similar times - they'll serve lunch after take-off and then breakfast prior to landing (click on this link and input "SQ51" to get a rough idea of the sorts of meals / drinks available):

https://inflightmenu.singaporeair.com/home

If you fancy a snack in between, just approach the cabin crew and they have a variety you can choose from and drinks are pretty much a free-flow from start to finish. You will also have a further meal served on the flight from SIN-BKK.

As for seating, assuming it's in EY then it's certainly worth contacting them to ensure that you're seated together - currently there's a lot of complaints being posted online about families / groups inexplicably being reassigned seats apart from each prior to departure, so it's best to keep checking to ensure that does not happen.