Hi Conal, we are in Felpham right by the sea but haven't joined a local club here yet since we moved down earlier in the year. Our local is Tone Zone (terrible name!) but we know lots of the Tone Zoners and Chi Runners through Bognor parkrun and they all seem very friendly. I guess it depends whether you end up Chi side or Brighton side...but come and see us at Bognor parkrun (there's one in Chi now too) From what I can see Chi would be your strongest road running club - I think even James Baker is a Chi Runner (google him, prolific race winner round these parts!).
Hattie, don't discount USA on cost - some of the places you've listed have incredibly expensive accoms and food...as we found in Switzerland in '09! So sometimes even though it's cheaper to get somewhere, being there is crazy pennies.