Where have you been looking? As far as I'm aware there aren't any restrictions on N2 in the UK, it's just more of an industrial product so it's sold at supply houses that don't market directly to the general public.
You can generally find nitrogen at gas suppliers and welding supply shops. The gas suppliers often cater to higher volume businesses and might not sell directly to consumers, although many do. The easier approach might be to look at welding supply shops, because nitrogen is a standard welding gas and it will be sold in reasonable sized cylinders because welders generally like to have a portable setup.
I did a Google search for "welding supply nitrogen" in the UK and found numerous online shops, some of which even have nitrogen cylinders in stock ready to add to the cart and purchase. You also might find it locally or nearby, since any decent sized city is likely to have a welding supply shop. Even if they only stock welding equipment and not gas, they still should know where to find welding gases locally.
The only concern a welding shop is likely to have about selling nitrogen to a consumer is that you're prepared to transport it safely. The cylinder is under very high pressure and it will turn into a rocket if the valve were to break off.
Finding the regulator should be as simple as going to Amazon or other big online retailer. You can also get regulators at a welding supply shop, but it will be more expensive. Those places make their biggest profits on accessories like that, and they also usually sell higher quality gear intended for professional use.
Nitrogen flow regulators aren't common, so it's easier to find an argon flow regulator with built-in pressure regulator at an online retailer. You can convert from argon to nitrogen flow rates using a conversion table which I posted in this thread:
I just thought I'd share a description of the gear I put together for inert gas asphyxiation with nitrogen and an exit bag. It's pretty basic since I felt it would be better to start with something simple and functional, then upgrade to SCBA/SCUBA or something else in the future if I have time...
sanctioned-suicide.net