Many thanks BoeingBoy and TheOddOne in particular for sharing your experience regarding RAeS membership.
My concern really wasn't so much about cost, I just found that RAeS host many events etc that I am rather interested in, whereas the benefits of AOPA UK membership were less clear to me (I had mixed feedback also from the instructors at my school sadly).
Your responses make the situation clearer, and I will definitely contact them with respect to GA in the Americas.