You should be same size as rain, but keep in mind some people go half size down some take same size as there carmina rain. In this case it's a boot if you do go 8.5UK you can still use thicker sock & you will have better idea of your actual size for future purchase. Hope this helps & this is towards EB boot on 809 last you cannot go wrong. Hope this helps.
Can you take a pic of the boots being worn?