I claimed CB as I was earning under the threshold for a few years when they brought in the higher income limit, and DH was the higher earner and earning just over. He did tax returns and paid the tax. Then I went back onto PAYE work in about 2014-2016 and had several short term/part time contracts for a couple of years before getting a permanent job, which took me over the higher earning threshold as well as DH.
HMRC actually wrote to me at this time and told me I didn't need to do tax returns any longer. As far as I was concerned, DH was doing tax returns in this period (though all of his income is PAYE) which reflected the CB, I was on PAYE and so the government knew that a) I was married to DH and he was accounting for the tax b) exactly how much I earned and claimed in CB without my needing to tell them.
Until the end of 2020, when HMRC wrote to me saying I owed about £1800 in tax from five years ago when for two years I claimed CB and was over the threshold. As apparently DH hadn't done tax returns those two years after all as they also told him he didn't need to. DH also works for the government and has high level security clearance so they know what he eats for breakfast never mind what he earns and that we are married.
It was an absolutely shitshow. I did pay it and only avoided paying penalties on top as I wrote them a cracking polite fuck off letter explaining all of the above.
What was particularly awful as well was they kept recalculating what I owed without telling me. So every time I logged into my HMRC account for a couple of months there was another £100 to pay. With a deadline to pay of the following week, which I would have missed if I hadn't happened to log in and check.
That and the way they totally fucked up my DM's tax affairs (pensioner and widow barely into 22% tax threshold) keeping writing to her saying she owes money or has not paid amounts she knew she had has made me totally lose confidence in them.
Also I have, thanks to that experience, a constant low level of anxiety, in spite of being in a permanent job and only PAYE and no CB for the last seven years, that they will write to me at some point in the future and say I have done something wrong and owe thousands of pounds.
My NI is actually ok but at the cost of losing all my savings from not travelling in 2020.
I have missing years from university time but I will have plenty of years by the time I retire.