Only Chubb sell Chubb branded extinguishers, which are an exclusive design to them. You must go direct to Chubb to get an identical replacement - however, you may find the price very high (£150 sort of area) if you are buying independantly - national & government contracts will get a more preferable rate, if you are a large concern you need to get your account details to get the cheaper rate (unless of course they pay top price - many large customers still pay list price).
Chubb own a trade subsidiary Thomas Glover, used by many independent fire protection companies & websites, where a kitemarked equivalent is available, but it is a slightly different design (identical fire fighting capacity though). Also Homesaver's FireChief brand is simply relabelled Thomas Glover extinguishers (they didn't used to be, suggesting that like the long defunct Allied Safety they have been bought out by UTC, Chubb/TG' global parent).
Waiting for the annual service is an option, but nationals suffer from poor facilities depts as missing, condemned & out of test extinguishers can often remain in place for some time whilst Chubb await approval to replace. This isn't helped by Chubb engineers not pointing out to each site's manager when they have condemned extinguishers so there is no one to 'push' facilities to hurry up with the approvals.
If you just want an extinguisher quickly you will find multiple suppliers who are far far cheaper than Chubb's list price in the Yellow Pages and web (Staples & Makro now sell a range of TG extinguishers too), but if it must be a genuine Chubb EF6, then it's Chubb you must go to