Most providers offer the service discount n their banking apps.
Camera on phone takes a form of PDF scan and applies it to your account.
Then post it to your bank.
Some credit as soon as the scan hits the account, some wait the normal 7days.. depends on your provider.
For instance: Barclays allow the scan, 7 days to credit the account.
Starling allow the scan and credit immediately (Starling are a Nat West subsidiary btw)
You can also scan cards and transfer money as well, again same principle but limited to £250pd.