Consumer affairs did a spot check on my web site. They bought something by CC and retrurned it a few days later. I got a list of a few things to correct and one was to declare my vat no on the web site. I argued it wasn't safe to do this on the net as anyone could use it. It is now on the receipt they get, not on the main site.
I wouldn't put it on my letterheads, only receipts or ivoices