I suggest to
copy files and related data of subdomain to somewhere
delete subdomain
create add-on domain test.my-domain.co.uk
Now you should have ability to create aliases for test.my-domain.co.uk.
[update]
You are able create subdomain test.my-domain.co.uk and add-on domain test.my-domain.co.uk. The difference in fact that for add-on domain you will have ability to create domain aliases.