Essential oils have been around for thousands of years, aromatherapists have not, now unless the laws have been changed then essential oils are available to everyone to be used as they see fit.
As far as I am aware the title aromatherapist has not been protected, so in effect anyone can set themselves up as an aromatherapist without formal qualifications, what they should not do is advertise themselves as qualified aromatherapists, as that would be misleading and illegal.
The legal differences between a qualified aromatherapist and a lay aromatherapist or any type of therapist is one of accountability, trained therapists are deemed to be accountable because of their training, lay people are not.
The other technicality is trained people because of their accountability need insurance to practise, lay people do not, though public liability might be a useful thing to have.
So can a massage therapist use essential oils to enhance their massage, if used sensibly and in an appropriate dilution and not used to treat dysfunctions then they should not have any problems, some insurance companies are covering themselves on this one.
So qualified massage therapists need to check with their insurance companies as some will and some won't but you need to define the difference between what you are doing as in not treating dysfunctions with essential oils as opposed to an aromatherapist who does use the essential oils to treat dysfunctions.