So, why a flow meter with nitrogen? That's not something common to home brewing - unless you're into nitrogen-purging brew rigs for low-dissolved-oxygen-brewing purposes
Anyway...you'd be looking at a welding style gas regulator with whatever cylinder coupler is required for nitrogen/mixed-gas cylinders...
I'd love to, but the standards and manufacturers Over There differ from Stateside. For example, I'd be looking for a regulator with a "CGA 580" cylinder coupler, while you'll be looking for one with a "BS 3" stem.
In any case, I expect what you're looking for would look something like this, very similar to my MIG welding shielding gas setup (as well as the O2 reg I use for aerating my brews, come to think of it).