We have made a Post how to get reservation :)

You can reserve Eurostar either on Interrail/Eurail website , on B-Europe (link in the post) or via Phone of Belgian or French Rail :)
 

Hey there, Thank you for the reply! 

i’ve checked both websites and none of them are giving me options to book a train from london to anywhere! 

Do you have a contact number that’s best to ring them as i’v tried to ring the help line but it takes me to an answering machine then cuts out! 

Thank you

Which trains(s) would you like to book then? Please specify route, date, departure time.

If it's for London to Paris, you can check availability here but don't count on it for at least the next 2 weeks since Eurostar sells out fast this summer. There may sometimes be a bit more availability to Lille or Brussels. Keep clicking "Later trains” to find the first available seats.

You can book via SNCF (press #85 for English), NS or by using the B-Europe site (but that charges an extra 4€ per order).