Question Author
Thanks for your help, Buildersmate. I've checked your link and it looks like a posher version of the conversion plug I have already. My problem is the waterpik needs to be used next to a sink (in reply to your question, a waterpik is like what they use at the dentist when the tube squirts water at pressure to give your teeth a proper clean. It's meant to be better than flossing). The cord it comes with isn't that long, probably about the same as an electric toothbrush, so even with the converter, I am still having to plug it into a UK extension plug and drag it into the bathroom which is not ideal, especially when water is involved.
I don't think there's going to be any solution to this. I was hoping the French plug would be the same as a shaver plug but unfortunately, the prongs on the French plug are slightly thinner so isn't a snug enough fit in my shaver socket.