Hi Graham
I can help regarding Eurostar as I'm responsible in getting our allocations. With Standard Premier, it rather depends how many people choose to upgrade but usually I try and get people sat in an individual window seat unless there are comments to the contrary. Eurostar has 4 types of Standard Premier seating - namely Club 4, Club 2, Duo and Solo. Club 4 (table for 4) is reserved for groups of 3-4 people, not allowing a booking for 2 to sit there unless they've ran out of Club 2 (table for 2) or Duo (2 seats side by side (called a twin also)). The 'solo' seats are good because you've plenty of space to put your legs out in front, underneath the seat infront, and you've an aisle one side, window the other.
http://www.eurostar.com/uk-en/travel-information/on-board/eurostar-seating-plan (where I note they've renamed duo to twin seating).
As you're in Standard Premier, you're probably aware that you'll get a 'lite bites' meal included in the upgrade costs. If you have any special meal requirements, please make sure that you advise us so that we can book it appropriately. We can specify things like gluten free, vegan, vegetarian, etc. not precise meals.
I hope that helps.
Kind regards
David