I have a pacemaker and I just told them I had one and they used a hand held scanner on me rather than going through the body scanner. I have my pacemaker card but they didn't even ask to see it. I did walk through the scanner on the cruise ship by accident but had no ill effects from it.Can't help regarding Eurostar, but I had no problems flying.
I have a pacemaker. At the airport I just show my St. June card and they use a wand instead. If you're not a frequent traveler it should be ok to go the usual way though (as you would going into a shop) but you shouldn't linger for too long.
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