I Googled 'Fire resistant greases' and got the ESSO website fairly quickly. Why don't you contact the larger lubricant manufacturer helpdesks (like ESSO, SHELL, TOTAL, MOBIL etc?) One of them is bound to have what you are looking for in their portfolio. Or is availability in the UK the issue?
Usually the 'big boys' will help specify the right product, and generally e-mail you the data sheets and any relevant health and safety data immediately, and advise you of your local stockist.
I would also recommend KLUBER Lubricant in Halifax, I normaly get sorted by Sandy McClure. If he can't help he wil probably be able to point you in the right direction.
Lester Milton
Telford, Shropshire, UK