Look here, each grade has several salary 'bands'
http://www.payscale.com/research/UK/Job=Teaching_Assistant_(TA)/Salary
As a teaching assistant rather than a fully qualified teacher your salary band is very narrow.
A qualified teacher has a much wider salary band so they will rise up the scale the more years experience they have. A teaching assistant has a much narrower salary band so the number of years experience will have little or no effect on salary. After 13 years you are almost certainly on the maximum possible salary for a teaching assistant.
The only way you are going to get more pay is to become a fully qualified teacher. I think you can do this by having 20 years experience as an assistant.
Maydup, sorry you are wrong. if qualifications made no difference to salary there would be no point in going to University and getting a degree or studying for higher qualifications.