I think the main point there is that WMUK office needs to remind people every time their membership is supposed to expire, with concise instructions about how to renew their membership.

 

If they do exceed the grace period, then email them again, this time simply saying their membership has expired, also with instructions on how to pay their fees and resume their membership. If they don't renew it, then, bye.

 

With these two in place I think we've got all bases covered :)