Not sure about lowland teams but for SARDA with the mountain teams you need to be a full member of the rescue team before they will consider you for a dog handler. - you obviously need to know how the teams work on call outs. In my area you have to have a probationary period before becoming a full team member. So you're looking at 18 months to 2 years. Then you have to get backing of Team Leader to be considered for dog handling & without that backing SARDA wont consider you. Some teams dont want dog handlers at all, i know of a few lowland teams this applies to.
Assuming all the above has been got through, the next stage is the dog. It's extremely rare for an adult fog to be taken into training. Most handlers will get pups, those pups are showing v distinct traits at a v early age, even from the point of entering the world.
Lots of assessments to do, many handlers never make the grade!
http://www.sardaengland.org.uk/There'll be links on this site to regional and national teams.
It's by no means an easy road www.sarda.org will give you more info.