Tying this together with your other thread, once you've been here 7 years, you will need to declare your US pension(s) to HMRC on a return or pay £30,000 to stay Non-dom. I gather from your other thread, you do not wish to pay the £30,000. HMRC operates your return on two numbers. Your NIN for taxes and a UTR which is your HMRC account number. There is a third number, the Employer Number, which you will not have need of. Your application for a NIN would be made on the basis of your 7 year residency, being a US not UK citizen, and need to file with HMRC on the arising basis.