What does it cost? £20
The official price on GOV.UK is £20, paid once for 2 years.
How long is it valid? 2 years
Or until your passport expires, if that comes first. It covers multiple trips.
How long does it take? Usually 1 day
Allow up to 3 working days and do not leave it to the last minute.
What if it is refused?
There is no appeal. You would then need to apply for a visa that matches the purpose of your trip.
Do children need one?
Yes. Everyone, including infants and children, needs their own authorisation.