How to make interrail reservations?
Rail booking websites usually only sell reservations at the same time as tickets. So you will have to book at a station or over the phone. You used to be able to do this at some UK stations but European reservations did not continue after the breakup of British Rail. If you are passing St Pancras you could try the Eurostar booking office. Most European railway booking systems are connected, so I suggest that you make your reservations on the first day of your trip. If you need to book some in advance, you can do so by phone (08448 484 064), but there is an absurd £8 reservation fee.