Terms not always upheld

Hi I hope someone is able to offer advice. I have been a full member of my gym for 15 years always paying full membership (not cheap). My daughter started working there in June 17 we found out in Jan I could have a cheaper membership on her family & friends discount. I gave the standard 3 months notice to move to this in Jan so would have moved to new membership May 18. Recently got a call saying I would have to leave for 3 months prior to moving to the family & friends deal. When I queried this as I am aware other members have moved straight over the manager said although the rule had not been enforced previously at all times his hands were tied and he refused to budge. I really do not want to leave for 3 months, do I have any legal standing as this rule by the managers admittance has not always been enforced? He is a new manager and it seems whereas previous managers ignored the rule he will not. He has asked me to put in writing my dissatisfaction and he will forward it to head office. It does seem crazy when they are refusing for me to be a member there for 3 months. Are there any consumer laws or any advice that would help me in this situation?