System Restore can return your computer system files and programs to a time when everything was working fine, so use this Windows feature through the Command Prompt to get rid of the Metropolitan Police lock screen.
Ensure that your computer is fully switched off - not in Sleep or Hibernation.
Restart your computer, and immediately tap the F8 key continuously until you get the Advanced Boot Options screen.
Use the arrow keys to select the Safe mode with Command prompt option.
If you get a User selection screen then select a user who has "Administrator" status (and log in with the correct password if necessary).
At the command prompt, type cd restore, and then press ENTER.
Next type rstrui.exe , and then press ENTER
After a short wait the System Restore window will open.
You may want to select the option to show more restore points
Select a restore point previous to the infection and left click the "Next" button.
After System Restore has completed you should reboot Windows in the normal way, then perform a system scan with the free version of Malwarebytes Anti-Malware.
(To save time, delete the Temporary Internet Files before you run this scan.)