Hi Septic
I would reccomend Plas-Y-Brenin and Glenmore Lodge also. However as has been said you may only gain a training qualification and not an assesment qualification from the short courses avalible.
Basically you can't take people out on your own until you have the approprite
assesment under your belt.
An intresting site with lots of info on outdoor stuff is
www.bluedome.co.uk . Many outdoor centres advertise for instructors here and it is possible to 'earn and learn' with some companies. The pay's probably going to be poor but you'll be gettting quality log book experience.
Glenmore Lodge also do the 'Night Porter' scheme- your employed at the Lodge in an ancillary role but when the a 'body' is required for an assement (climbing for example) you go an act as a non experienced client to the person being assesed.
Be very wary of centres offering outdoor diploma's, they dont count for much. As an outdoor instructor the usual qualifications you need are Mountain Leader (summer) Single Pitch Award and BCU Level two in canoing/kyacking.
Ed