Don't feel bad, pengwen, I made the same mistake. When I first moved here, I was living in B&B's and off fast food. After a week of greasy take-out rubbish, I was seriously jonesing for a simple ham and cheese sandwich with mustard.
Went to the little village store, but didn't recognise any of the mustard. Only two choices: English or French. Being some what patriotic to my new country I decided on the English mustard. Went home and slathered it all over my sandwich - as you do with French's - and had the exact same reaction you did. I think I guzzled about two pints of water just to cool my mouth off.
Never fear, tho. French's can be found here. I've seen it in both my local Tesco and Sainsburys.