Belgium
I would put Belgian pralines on the list for Belgium because it is the kind of thing that feels useful once you are home, not just another airport purchase. Look for a boxed version with a decent date on it, especially if you are buying it near the start of the trip rather than at the airport. The reason it works is that it gives you a quick reminder of the trip without needing to keep a fragile ornament safe forever.
Belgium
When I think of an easy, crowd-pleasing thing to bring back from Belgium, Belgian beer gift pack is a good pick because it does not need much explanation when you hand it over. For alcohol, I would only bring this back if it is sealed properly and comfortably within the UK allowance; otherwise it is better as a UK-buy link. I would rate it higher when it is locally made, clearly labelled and easy to use at home, and lower when it is just tourist packaging.
Belgium
This is exactly the sort of thing I like bringing back from Belgium: small enough to fit in a bag, but specific enough to remind you where you bought it. Biscuits and dry breads are good because they survive the journey and are easy to share at work or with family. It is also the kind of thing you can talk about when someone asks what you brought back, because there is usually a little story attached to where you found it.
Belgium
Lace bookmark is worth adding because it feels like something you actually noticed while travelling through Belgium, rather than a generic souvenir. The trick is to choose the simple, well-made version. Small craft items look better on a shelf when they are not too shiny or mass-produced. The ideal version is modestly priced, not too bulky, and good enough that you would buy it again even without the holiday memory.