Welcome and well done for planning ahead. Much easier than sorting out problems later. There is loads of advice in the poultry info section and it is definately worth reading through.
To answer a couple of questions:
Yes the chickens (or really the feed) can attract rats so keep things clean and be prepared to deal with any infestations that might occur.
No you don't need a liscence but some properties have limitations in the deeds as to whether you can keep poultry.
yes they will clear 4 foot in a heart beat, probably even if wingclipped!
(Have you thought about ducks
![wink ;)](https://chat.allotment-garden.org/Smileys/green/wink.gif)
, they wouldn't be anywhere near as likely to!)