Go to www.census.gov.uk to fill in your form, and you will come to Question 15, "How would you describe your national identity?" where you are invited to Tick all that apply:
 Northern Irish
 Other, write in
Are you going to write in "United Kingdomish"?
If that makes you smile, that's because you know it's absurd to suppose that the UK is a nation. It may provide passports but it does not provide identities. That must be because it only came into being under a hundred years ago as a result of an Irish independence movement which only half achieved its aims. Our rulers hung on to six counties of the colony we had created in Ireland and tacked them on to Great Britain.
Are you going to write in "Great British"?
Well, that would be immodest even if "Great" historically has the meaning of "Greater" - ie, "larger" as in "Greater London"
Our Westminster politicians are very much hoping that, unless you are "Northern Irish" [is that really a "national identity"?], you are going to tick English and British, Welsh and British, Scottish and British - and best of all, just British [ and best]
I don't want to give any satisfaction to our Westminster politicians. I will tick English. I was born in Kent, went to school there, and have for most of my 63 years studied, lived or worked in England. My children grew up in England.
And I might write in "European" because I think the ideal of a Europe, free of military conflict, with open borders and a single currency, is inspiring, however awful the bureaucracy in Brussels may be. And in Europe, there are republics and countries with no Established Church.