You can find a list of UK distributors on the RossTech website here
https://www.ross-tech.com/distributors.phpThe software is free to download direct from the ross-tech website (
https://www.ross-tech.com/vcds/download/) , the license to use it is built into the VCDS dongle.
By registering the serial number of the dongle you get RossTech technical support and forum access - you _must_ register the dongle within something like the first 30 uses.
You
can buy second-hand dongles (watch for fakes as they won't work with the newer versions of the software) but bear in mind they will be registered to the previous person. i.e. They'll work fine, you can upgrade the software etc but you won't get forum/tech support unless you get it re-registered with ross-tech, this involves a $200 fee and the previous owner to be compliant.
As someone pointed out here you can get three types of license, a 3 VIN, 10 VIN and Unlimited VIN, the VIN count goes down with each new car you use it on and perform a 'major' operation e.g. an adaptation or a global clear of DTCs (fault-codes), so you can clear individual DTCs and do diag scans on your mates VW without using up a precious VIN slot.
As GVK mentioned, you can pay to upgrade your VIN count. It might not seem obvious but going from 3 to 10 doesn't give you 10 more VINs, it gives you 7 more, once you've gone to 10, you can upgrade again for the same price to unlimited. You must be the registered owner to do this.