Hi,12 to 18 months is a good time to start doing agility training ( sorry don't know anything about flyball ).I take Gabby to Mutts in Motion (part of Rixton Dog School ) which is in Warrington and the classes are run by Carolyn Snowdon who is very good. Carolyn is also the secretary of Wilmslow DTC. Dig It Dogs is also very good,and they run their own shows and have lots of fun club competitions.Also Newton Heath DTC has a good reputation but I have never trained there.
I think If you are interested in doing dog agility its a good idea to find a good trainer as opposed to a fun agility class,as they will start your dog over the correct height jumps and use safe equipment.