I wrote to Oulton about this (for a specific event), the requirements were that I had PLI as noted above, I had to produce some examples of my published work, and I had to produce a letter from a publication editor engaging me to photograph the event on their behalf. Being over 18 the PLI aspect wasn't really an issue (while the £5m sounds a lot, the policy cost isn't really that high) but the other two were more of a challenge, and I do wonder how a beginner would fare, as if you're trying to start out in these things you're not going to be able to produce either.
I had a conversation with a mate about this, and he pointed out that he'd been at Oulton recently and seen people with media bibs aged "around 16", so whether they do some kind of school-related thing or the person he saw was with a parent who had accreditation, I don't know. I can't even say how he established the persons age, but he did seem quite sure.