You don't need one yet. Only stage events require a national A and the only championships I can think of requiring a national A are the British Championship, The British Historic championship and there used to be a national tarmac and national gravel championship requiring them but I don't think they do any more. Also if you are planning to go to Ireland or Belgium you will need a national A. All single venue and many multi venue rallies run on a national B permit. There is also often a national B clubman event run alongside the national A event.

Of course there is no harm in you wanting to upgrade, its just a more expensive licence and you won't use it for a while I wouldn't think as you have said on here for now you are concentrating on road rallies and targas which will never need more than a non-race national B, so that doesn't even require a BARS test (which allows you to get a stage drivers licence).

To upgrade you need to have completed either four national B stage rallies or three national B stage rallies and your BARS test can count. You prove this by asking the clerk of the course (and BARS instructor) to sign an upgrade card which is supplied in your Blue Book, at the completion of each rally. Having said this I never actually got a signature for any of my rallies and the Motorsport UK accepted printed results from the web but it was a few years ago so that would be worth checking.

Also I hope you don't mind if I suggest you come and find one of us at a club night and ask as many questions as you want. That way it won't be piecemeal from individual posters.