Searching online, it looks legit. But wondering how they can provide refundable tickets while most others and the official site (lastminute.com) do not offer them.
Personally I would not buy London Eye tickets in advance. Fixing the time you go opens up the chance that the weather will not be great at that time. It's much better to choose a good time when you are in London. The queues can look long but they move very quickly.
GYG is a great resource for exactly that reason - cancellability. They charge the same price.
Do note the Eye will allow you to transfer your date and time for free within a time period (from memory 90 days) so okay if you know you are going some time, but not fully cancellable.
I use GYG all round the world for that flexibility.