You will need to prove your right to work in the UK to your employer before you start working for them. How you do this depends on your nationality and what kind of permission you have to work in the UK.
If you’re a British or Irish citizen, you can prove your right to work in the UK with either a British passport or an Irish passport or passport card. Your passport or passport card can be current or expired.
If you do not have a passport or passport card, you can prove your right to work with a UK birth or adoption certificate or an Irish birth or adoption certificate or a certificate of registration or naturalisation as a British citizen
You must also give your employer an official letter or document from a previous employer or a government agency. For example, you could use a letter from HM Revenue and Customs (HMRC), the Department for Work and Pensions (DWP) or the Social Security Agency in Northern Ireland. The letter must show your name and National Insurance number. If you have a passport or passport card, you may be able to use an online identity service provider (IDSP) to prove your right to work
If you’re not a British or Irish citizen, you can prove your right to work with a share code. You can choose which option you use. Your employer cannot reject your application because you gave them an eligible immigration document instead of a share code.
If you’re not a British or Irish citizen, your employer can check if you can work with the employer checking service