I posted on a tropical fish forum when I had to rehome a whole tank. I got three or four offers and chose the guy who had five tanks already and wanted a new one for his kids on the basis that he would know what he was doing.
It took about two hours to net and bag the buggers (fairly large tank of livebearers with lots of babies!) and he was very careful and patient, had brought a cool box with heat pads etc to transport them.
If you're offering them for free you can afford to be picky about who they go to. I'd want to be sure my fish weren't going to get flushed and the tank used for something else!
Have you thought about offering them to the local school? I know a lab tech who has a lot of tanks at her school (sadly nowhere near you) and they make a good "class pet" especially if you were able to go and help show them how to water change etc.
If you want them to go to a good home then your best bet is to sign up to a few fishy forums. There are always people with a bit of space that can help you out. I would offer but I'm not in Oxfordshire. What have you got?
We have a tank of tropical fish that we need to rehome, as we're moving house and can't take them with us (unfortunately!). Does anyone have any experience of this - I've seen a couple of charities and individuals on the internet offering to rehome fish but we want to make sure they are definitely going to a good home .. alternatively, if anyone would like to offer a few small-ish community fish a good home (we are in Oxfordshire) please feel free to PM me! Thanks