Yes you need a reservation (30€) for taking the Eurostar to/from London. You can book it via interrail.eu reservation service (+ 2€ fee/train) or B-Europe (+ 4€ fee/order), or at SNCF (France), SNCB/NMBS (Belgium) or NS (Netherlands) Ticket Counter. I hope I didn’t forget some other possibility to buy it. 

Thank you Angelo, so it's more convenient and cheapest a plane ticket....

OR a bus-FLIX or blabla-a normal ticket is most often cheaper as even that 30€, but the ride takes longer-often via a ferry

it depends, when you start in Paris, Brussel (only 2 hours from city center to city center) or Amsterdam the Eurostar is the best way to get to London, but not the cheapest (most of the time).