A degree in a related area such as marketing, IT or business and technology may be particularly useful.
Entry into graduate-level SEO/marketing roles with an HND, or foundation degree is possible but you will need to have substantial, relevant experience and technical skill.
Postgraduate qualifications in digital marketing may be helpful for expanding your skills and knowledge, particularly if you have a non-related first degree.
Other exception is that you are self taught and have been on the seo skin for quite a number of years. If you don’t have relevant qualification but have enough experience and past proven work then that should be somewhat enough.