Googled them + UK and found loads of crutches, but not sure if any are what you want. If you can get them abroad somewhere, can you order from them and get them delivered?
Alternatively is you know where to get them in France, and they don't deliver to UK, you could order them and get delivered to my friend just across the border, and I could bring them when we come late September/october. We will come via Dover to Guildford, then Dartmouth then Market Harborough. And if you can get them in Switzerland, get them delivered to us, and then as above.
Bonne chance.