You can always add trains separately in your travel diary, so simply add an earlier train towards London. Note the rail planner app is not very reliable for planning, as evident from this: 51 minutes before a Eurostar departure is too tight for comfort. 

I agree that it is too tight. Have 2h margin I'd say.

Please note that you need to add to book Eurostar ASAP, even more if you need to take a specific train. There is a quota for passholders and it can sell out weeks in advance, especially between London and Paris. 30€ seat reservation

Check availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish

Let us know if you need alternatives and we'll gladly help.

Night trains are very popular so book them ASAP if you'd like to take them.

ICE trains from Belgium have optional reservations but are very popular so if you don't make one (3€ through ÖBB), you'll surely have to stand.

Depends a bit on the day and how busy it'll be. But if you have to go from another London Terminal to St. Pancras (except maybe King's , that's indeed too tight.

What's your route, exact trains you'll be taking, travel date?