Once; on a training flight in a VC10 from Prestwick to Shannon when we were held down by ATC over Belfast at around 18,000ft (there was an ATC strike at the time). Ice began to build very rapidly on the windscreen wipers so I immediately put on the wing anti-ice and asked for a higher level which they refused. I argued and eventually got a higher level by saying I would go up anyway if they didn't coordinate with the next ATC facility. We got our higher level, but by then we had nearly 2 inches of ice on the wipers (they were a better ice detector than the official one!).