I am not familiar with UK sizes, but you pretty much have to try it on. If you buy a lot from the internet, check sizing charts, which are usually posted on the sites. If you try on a boot and it is too tight in the calf or thigh, you will start to feel your heartbeat in your calf or foot after about 5 minutes. You should be able to tell if it will fit you well or not if you put the shoe on and it feels too tight or too loose, try a different shoe. A shoe should be comfortable as soon as you put it on. That is the key to finding the correct size. You can find some websites like
www.snaz75.com that will show you the size conversion charts and you should also keep in mind that a wide foot and wide calf are usually going to be a little more snug in women's thgh boots.
Try
www.queenfashions.com www.janetscloset.com www.allheels4men.com or
www.crossdressers-windsor.co.uk www.doreenfashions.co.uk These sites should have some helpful information on selecting the right size or at least a size conversion chart or something