I was always under the impression that a passenger needed to be physically mobile to sit in an emergency exit - indeed I believe that CAA rules state that "..You must be a non-disabled adult in full fitness...". So I was a little surprised when a guy in a protective boot using crutches sat down in the other exit row immediately in front of me on an LHR-MRS flight last week. Does this requirement not actually get enforced any more, or does boot + crutches somehow count as "full fitness"???