OK, so having a bit of a google it seems it should be something along these lines: https://www.trade-tariff.service.gov.uk/headings/6210#notes which would be 12% duty, but digging into it a bit further "bullet proof vest" comes back as 6210400000 which is 0% as an import from the EU https://www.trade-tariff.service.gov.uk/commodities/6210400000#import
Again, it ENTIRELY depends on what they put on the CN22.