ISAs and savings
Each provider; ISAs may pass to a surviving spouse/civil partner by additional permitted subscription rules where applicable
Some steps differ by region inside the country.
Where
The ISA provider or platform, and NS&I for Premium Bonds and savings certificates.
How
Look for annual statements by post and in their email, these identify each provider.
Contact each ISA provider for the value at the date of death.
Contact NS&I separately for Premium Bonds, which people forget because they rarely generate statements.
If there is a surviving spouse, ask each provider about the Additional Permitted Subscription.
What to bring
A certified copy of the death certificate
Account or holder numbers
The grant of probate
This is practical information, not legal, medical, or financial advice.