Firstly hello and welcome
Im maddy and we have a 15 month old working strain cocker and she is the apple of our eye
No advice on a dog flap as I have never used one but I guess someone will be along shortly that has and can help. Willow likes to toilet where she can smell other dogs so for that reason I am guessing she prefers to go on grass, either that or in the spinny near where we live in amoungst the weeds etc. She will go else where if desperate but doesnt look happy about it.
I dont know re breeder of show types im afraid but I am guessing someone will be along soon with info to help you.
In relation to your fence, i know cockers can jump high, we have a 6 ft fence around 3 quarters of the garden and the other section a 5 foot fence, she has never escaped although when we first got her people advised us to replaced the shorter panels, this has never been an issue for us.
Maddy