If you are a British citizen, or entitled to British citizenship, and do not have a British passport, you must be able to show that you have the right of abode in the UK. You will not be permitted to enter the UK unless you have one of the following documents.
Certificate of Entitlement
If you are entitled to British citizenship but have not held a British passport, you can apply for a Certificate of Entitlement to the Right of Abode. The process involves completing the online application form, paying the fee (currently £388), and attended a visa application centre to complete your biometric enrolment. You can expect a decision within 3 weeks after your appointment. A successful application will result in a return of your foreign passport containing an endorsement vignette (sticker) confirming your right of abode in the UK. It will be valid for the same length as your passport, and must be renewed once you obtain a new passport.
Emergency Travel Document
If you are a British citizen whose passport is lost, damaged, or full, you can apply for an emergency travel document for urgent travel. To be eligible, you must be a British citizen who is outside the UK and needs to travel within 6 weeks. You must be unable to renew or replace your passport from abroad, and you must have held a valid British passport within the last 10 years. The process involves completing the online application form and paying the fee (currently £100). A successful application will result in an emergency travel document being issued within 2 working days after you applied. You may be required to attend an interview at your closest British embassy, high commission or consulate. Your travel document will be retained by border control upon entry to the UK.
If you are entitled to British passport or have previously held a British passport, the most straightforward and long-term solution for avoiding having to prove your right to enter or stay in the UK would be the acquisition of a new British passport. Your certificate of naturalisation or registration will be the main document needed for the acquisition of a British passport. There are other requirements which can be found here. It's worth noting that a person's eligibility for and entitlement to a British passport will be considered on a case by case basis, and is not an automatic right.
If you are unsure which of these options is most suitable for you, please contact us to discuss your situation.