Hi,
There's a good range of kneeling chairs here:
http://www.backinaction.co.uk/?action=kneeling
I have one that looks the same as the "posture chair" on their site, but I got mine from Argos years ago (looks like Argos don't do them any more). Not sure if I could sit on it all day, but it looks like they have some there that would suit that need better.
As for health and safety at work, it can depend on what work you are doing. I've never know that arms on a chair are an actual h&s requirement, but having the arms comfortable for the work you are doing is important, so if you're at a computer all day, having something to rest the elbows on while you type could be consider a h&s requirement. There are good office chairs now with good lumber support and adjustable heights and angles etc. to allow you to get the most comfort for you. Perhaps consider looking at those various models and then get your company to buy one for you.
All Love and Reiki Hugs