Air is much better. If you have a dive bottle, then you can fill your tank where and whenever you like (if that's filled too). I suppose with the proper filling and regulating equipment, you could fill at work, though I don't know how safe this would be and whether it would effect your insurance if something went wrong...
You can use HSE but not DOT bottles in the UK (I think that's the right way round).
There's loads of different air systems out there- some are fixed output, some adjustable. I've personally used Smart Parts max-flow- very good and consistent, and currently use an Angel Air- top quality and bling bling too...
Hope this helps