Are there any rules in the UK on this.?
The ruling here in the UK is the same at an overwing exit as in the States (with maybe slight variations). At the airline I work for here in the UK our ruling is; Able bodied passengers (ABP's) to be sat here i.e; no elderly, infirm or person's with reduced mobility, no children or under 16's, no obese, no deportees, person's in custody or prisoners, no ill or pregnant passengers/passengers with infants. To be fair the list is endless. The problem being that a lot of passengers know this and will do there best to make check-in unaware of some of the above and compromise the safety of others for their own selfishness of wanting extra legroom. In all fairness yes if spotted the crew should move and reseat this person if unsuitably allocated this seat by check-in which brings me to your comment.....
I really thought this was a serious ommision on the part of the crew
Would that not be check-in maybe??? Yes I agree the crew should have noticed this, but sometimes I wonder where the check-in staff's salary is on my wage slip cos I'm often doing the job they are paid to do in the first place when having to reseat and rectify their mistakes on board!