https://www.legislation.gov.uk/uksi/2015/17/schedule/1/made
For example you can use the characters w,i,n,d,s,o,r but catenated in an ordered string means you have to get special permission.
Arguably the attempt to register this was an attempt to breech the Computer Misuse Act (which would make it illegal).
https://www.legislation.gov.uk/uksi/2015/17/schedule/1/made