Are cargo bikes legal in the UK?
Yes, the same rules and regulations apply as a regular bicycle (you don't need insurance or a licence), and you can take them on the same routes you would any other day to day bicycle.
Are they difficult to ride?
Riding a cargo bike is not hard, but it is different than riding a regular bike. To give you some tips there are several videos on the Internet. This way you will definitely get the hang of it!