Well, looks like your only option (if you want to retain the French version) is to try to borrow an original French XP Home edition disk. Or, buy one - from eBay click here perhaps? Some will have just the disk (sans sticker / sans COA / sans whatever it is in French!) for sale, which should be a lot cheaper. Also enquire if they'll be willing to post to the UK before bidding/buying. A similar search on eBay UK didn't turn up anything click here, but keep checking from time to time.
Do you have the COA sticker/product key for the French version installed at present? If no, use one of these free 'keyfinders' click here to find out - use a couple of different ones for verification purposes, & make a careful note of it. G