Tawny owl is the is the most common owl in uk around 20000 pairs but has been declining. They are the only owl that makes the charactaristic twit-twoo - a duet " twit "by the male and "twoo "by the female, they mate for life..They are the most common in europe too.
The barn owl is one of the most widely spread and is to be found worldwide, but Skylight is right about the tawny owl being the commonest within the UK.