There are 2 excellent internationally recognised diploma courses affiliated to the International Federation of Aromatherapists (the longest established professional Aromatherapy organisation): The Aromatherapy Diploma (level 4 diploma) and The Professional Essential Oils Therapy (PEOT) Diploma which focuses on the in-depth study of essential oils and carrier oils and their applications, (without the Aromatherapy massage).
Here is a link for further details:
http://www.ifaroma.org/index.php?cID=2915The website gives details of all their approved training centres worldwide, including London, one of which is The Spa Isha Academy based in West Ealing.