My wife uses a wheelchair, and we travelled London - Frankfurt last year.
Post 2 is correct as to pricing - which I know is all you asked about. However, have to say ES were great with help - boarded us onto the train first at both ends, met us at Brussels on the outwards and return, and even pushed her wheelchair through to the interchange platform. They also stuck us in a cab when the lifts had broken down at St Pancras so we could make our onward connection.
DB were a bit spottier, but no complaints (couldn't find anyone to help me board at Frankfurt, but not a big problem as all the other passengers helped, which was nice of them). Make sure you put the wheelchair in the "official" spot for it, as I didn't know there was one and got told off.
When booking, I called DB directly (they have a UK number) and booked with them, including the ES part. They then organised all the assistance for me, much easier than trying to do it online.
Hope this is useful,
Drew
