Several points. You might find the website "Rescue Review" helpful link. This gives updates on people's experiences with different rescue organisations.
It's also helpful to have a quick update on what sort of rescues you might encounter. Firstly, there are the big national organisations like Dogs Trust and the Blue Cross. They have kennels all over the country, and usually have very thorough homing procedures. They will supply back up - only disadvantage may be that as a large, national organisation, their policies can be a bit fixed. But definitely worth a look if you fit their criteria.
Secondly, you have breed rescues (often national for less popular breeds, or regional for more common breeds such as Labradors or greyhounds). These specialise in one particular breed, so are perfect if you are wanting a very particular sort of dog, and many are very well known. Examples would include Wiccaweys (Border Collies) or Scruples Whippet Rescue. The beauty of these sorts of rescues is that you will unparalled access to real expertise in your particular breed, and for many people it also becomes a community of interest as well. For specialist breeds (huskies spring to mind) they can provide a great deal of advice and support for activities such as racing, sledding etc.
Thirdly, you have national rescues that may have other specialities - the wonderful Oldies Club springs to mind, which as the name suggests, finds homes for older dogs.
Fourthly, nearly every area will have a few all-breed rescues. Some will take more than one species, many will take dogs from local pounds, or other sorts of relinquishment. Bear in mind that they are NOT subject to any minimal standards, licensing or independent inspection - standards do vary wildly. Good ones will:- insist on a thorough homecheck, will carefully match you with a dog (not just one who you have fallen in love with), may be prepared to say no, will give FREE ongoing support and behavioural advice for the duration of the dog's life, will always be willing to take the dog back if you have an emergency/change of circumstances, will insist on the dog being neutered, chipped, wormed and thoroughly health assessed before homing, will tell you about any health or behaviour issues that are known, and will offer lots and lots of lovely social events later on, so you can meet and socialise with other proud adopters. Most all breed rescues will do their utmost to make sure that a dog has been in the care of a foster home before adoption - this gives a much more accurate picture of the dog's character and behaviour, especially around children, other pets etc.
Lastly, there are pounds. Basically, this is where unclaimed strays end up. Generally, you walk in, say "I'll have that one", pay and walk out. No homecheck, no aftercare, impossible to do accurate assessments. Some are better than others, but this is NOT the route to go down. Mostly, rescues try to work with pounds to get the dogs out before they are killed, then they can work to assess them and get them rehomed properly. Pounds tend to have higher rates of dog returning and the outlook for returned dogs is not at all good. 
Hope this helps.