I have never been to Kestrels but I have been to Ab Fabs both clubs are run at the same premises but at different days and times of the week. AbFabs is great if you have a swinger partner to go with. If you go alone on days that single men are allowed, you can get lucky and sometimes you will be asked to join a threesome or a gangbang.
Rios is a different matter. Although there is some action it's very sporadic and not as much as in AbFabs. Sometimes, and I mean sometimes you will get lucky. It gets embarrassing at times though when a hottie goes along and hordes of horny men start following her around. I used to go on Tuesdays when it has a 'naturist' theme i.e. no swim suits allowed everyone has to be naked, if you wear a swimsuit they actually ask you to take it off or leave but most people wrap a towel around themselves.
On Tuesday evenings I used to bump into this amazing MILF with massive knockers who used to come with a different toyboy every week. Personally I never expect to get some action at Rios unless I go there with my FB. When I went on my own I used to go and visit some prossies in Kentish Town first (there used to be a nice polish group there once) and then go to Rios to relax in the jacuzzi... it worked for me
For more in-depth info you need to check swingers sites really, as there are hardly any prosssies going to spas - unless you book one to take there yourself.