The Eurostar Business Premier lounges in London, Paris and Brussels offer a very similar selection in my experience and this selection varies depending on the time of day. In the morning, pastries are offered by way of a continental breakfast (toast is also available on request) later in the day from lunchtime until early evening sandwiches are usually offered and later in the evening just some Japanese rice crackers and nuts etc.
On the beverage side, this is the same throughout the day, coffee machine with good selection of herbal tea bags, small cans of coke etc and bottles of water (useful to pop in your bag if you’re travelling in standard) good selection of wines, spirits and beers.
Generally I’ve always found the selection in the lounge sufficient, if you’re travelling in Business Premier or Leisure Select then you’ll be served a meal and bar service on board and if you’re in standard then it will tide you over for the journey. And every time I escape the rabble at Gare du Nord for the sanctuary of the lounge it makes every penny of the Amex annual fee worth it!