The VIN plate kindly provided by Adrian went into my chassis this week.
I decided to stick it on the inside of U6, which I think is ok.
Rather than weld the plate onto the face of U6, which Mr IVA Man may or may not approve of, I got the angle grinder out and cut a section out of U6 the same size as the plate. Then I welded it in and ground/sanded down the welds so it's nice and flush, and will hopefully (once I've finished blending it in) look like the number is engraved straight into the chassis member