Birth certificates
General Register Office (England & Wales); National Records of Scotland; General Register Office Northern Ireland
Some steps differ by region inside the country.
Where
The General Register Office, or the register office for the district where the birth was registered.
How
Check their own paperwork first, the original is often already filed with the will.
Order a replacement online through GOV.UK, or from the local register office.
You will need it to prove identity and relationships for pensions, benefits, and some probate applications.
Order copies for a surviving spouse and children too if they are needed to prove entitlement.
What to bring
Their full name at birth, date and place of birth
Their parents' names, including the mother's maiden name
Payment for each copy
Sources
This is practical information, not legal, medical, or financial advice.