They did register their domain name about two and a half years before yours, which may be when they chose their company name, or may just be when they bothered to start setting up a website. Find out when they started using their mark, and compare to when you started to use yours. Do you have any proof of date of first use? Did you register your mark?
Otherwise, they're in Britain, you're in the U.S., the colors aren't even close, it doesn't look like they've registered their mark here, it isn't famous, and unless they've been selling in the U.S., their mark is junior to yours here (at least in the region in which you've been selling your products). Demand that they retract their defamatory letter, shutter their doors, and pay you a huge wad of money. I also suggest reminding them that we took the San Juan Islands from them after killing just one British pig, and we can do the same again in bloody Shropshire. Good cheese, though.