Might sound a simple suggestion but does a hyphen help somewhere in the domain name? You are aware that a domain name has no legal connection to a business in the UK. The only circumstances that Nominet (UK domain registrars) are interested in is domain squatting. I.e someone deliberately buys a name that they have no legitimate use for and then they try to sell it to you at an overly inflated price
You have half a point in that Nominet won't be particularly helpful/interested in the actual initial act of passing off; it's not them you have to worry about, but action taken via the courts. - In marketing terms it's also both lame and shoddy to be too close to someone else's trading style.