To install the 'United Kingdom Extended' keyboard:
Control Panel > Region and Language > 'Keyboards and Languages' tab > click 'Change keyboards...'
In the 'Text Services and Input Languages' window, click 'Add...'
In the 'Add Input Language' window scroll down to 'English (United Kingdom). 'United Kingdom' should already be ticked, and under that should be 'United Kingdom Extended'. Tick this, click 'OK'.
Now back at the 'Text Services and Input Languages' window select 'English (United Kingdom) - United Kingdom Extended' as the 'Default input language', click 'OK'.
Now back at the 'Region and Language' window, click 'OK'
The above is for Windows 7, but Windows 10 should be similar