Speaking as a puppy walker and having also found out all about rehoming a guide dog as I wanted to do it myself, lets just say that there are lots of hoops to jump through. There are different categories of rehoming (although this is just what I found out, Im not employed by them):
A retired guide dog, these are like gold dust, as they generally stay in the family where they have been living for the last 7ish years, after all if a dog had been your eyes for that length of time you wouldnt want to lose touch with them. They can also be retired because they have developed arthritis or other health problems. I shouldnt think they would place a dog of that age with an young family just due to the fact that they would want them to have a quiet retirement.
A younger retired guide dog, this could also be due to health or behavioural issues, they would try and either find an alternative career for them or they would be offered back to the puppy walker first.
A pup rejected in training, this is more normally due to health or behavioural issues such as development of HD, eye problems, extreme skin problems or IBS. Fear of harness/severe distractibility/aggression towards other dogs. They would still try and find them an alternative career first, and Guide Dogs have also bought in something called Buddy Dogs which they are running as a pilot.
http://www.guidedogs.org.uk/adviceandservices/buddy-dogs/ which are a good use of the dogs that arent quite up to Guide Dogs but are still fantastic in any other way.
Pups rejected at walk, due to behavioural issues such as aggression etc or health problems that are severe, one of my fellow puppy walkers started to walk a pup that was discovered to be blind in one eye. She kept him!
The dogs are always offered back to their puppy walkers first, (well that is what is meant to happen anyway) and then there is a long waiting list for the others. So you can see that the dogs that are available for rehoming arent necessarily what you were thinking of, plus if you narrow it down by only asking for a girl, or even just a colour you will be waiting longer. There is no longer a central pool of dogs that are available, you also have to wait for one to be available in your area now (thats changed recently). Ive been on the waiting list for over a year and Ive not even heard anything at all. You are now expected to pay for all the health issues that the dog has (that has also changed as previously if a dog came with health problems then GD would cover it). You dont have any guidelines to follow (so not sure where the obedience/training thing came from) only that you dont use the dog for guide dog fundraising unless you are a registered volunteer.