Both Capitaine Train and Loco2 work pretty well, normally much better than the websites of the companies who run the trains! Both are run by startups trying to shake up the international rail booking process. Neither has booking fees.

Main difference is that Loco2 sells in GBP, while Capitaine Train sells in EUR. Loco2 can also sell RENFE tickets for Spain along with UK domestic train tickets, which IIRC Capitaine Train can't. Capitaine Train supports things like SNCF loyalty cards, which tend to only matter for very frequent travellers, though I'm told that Loco2 should offer that too fairly soon. Oh, and Capitaine Train offer discounts off your first trip booked - this signup link will get you €10 off your first booking.