Volunteers also don't need a licence, although I'm not sure if being reimbursed for travel expenses could be counted as payment. That would be for the courts to decide if it ever comes to that (and if it did the site would probably have bigger problems to worry about)
It could also be argued that marshals are there in more of a stewarding role than security so again wouldn't need to be licensed.
More info on what you do and don't need a SIA licence for can be found here
https://www.gov.uk/guidance/find-out-if-you-need-an-sia-licence#when-you-do-not-need-an-sia-licence
Its not so much the cameras being worn which could be a legal minefield, more how the images are used
Has anyone asked Driver Wood why they've decided to do this?