Lexity:
I have to travel from Doncaster to Belgium at weekends quite frequently, but after 1600 on the Friday, and can take the East Coast Express/Eurostar to Brussels or Trans-Pennine Express/Citiflyer (VLM) to Antwerp.
Having done both, the latter method wins every time. Both British trains are equally crowded; the legroom in the Eurostar is poor at tables and the buffet dear, the queues at both St Pancras and Brussel Zuid are terrible; this is compared with short queues and check-in times at both T3 and Antwerp, and excellent free service by VLM. Sometimes I have to return by Brussels Airways, but Zaventem is preferable to Brussel Zuid.
You can bring bottles/beer back on the train; but last week I checked what should be a hand-luggage rucksack into the plane hold because I had a special bottle of wine; the staff kindly labelled it 'fragile', and it was on the belt at Manchester five minues after arrival in the luggage hall.
The only problem with the Antwerp flight is that sometimes they seem to get a block booking on the Fokker, which sends the price sky-high. So in March it quotes a return of £215, while the weekends either side are £86. Regretfully I'll have to take the Eurostar that weekend at c.£120.