I assume that you know of the Aber tri club. If not, then here they are:
http://www.ceristtriathlon.org.uk/index.htm
They run an aquathlon series every spring, one of my sisters took part a couple of years ago and enjoyed herself; in summer they have a triathlon but I somehow think that the bike ride involves hills.

I can also ask the other sister which tris she has done and which are the most fun.

How about the Timex WomenOnly tri at Dorney lake?
http://www.humanrace.co.uk/more-info...thlon-404.html

Dorney lake is just off the M4 near Eton, so it shouldn't be too hard for any of you to get to.