Over 100 straits including Dover strait become territorial sea as a consequence of extending the limit of the territorial sea to 12 miles.
In these straits, foreign flag vessels have the “ right of transit passage”.Transit passage means the exercise of freedom of navigation solely for the purpose of expeditious transit of the strait between one part high sea or EEZ to another part of high sea/EEZ.
During transit, passage vessels must: proceed without delay refrain from the use of force or threat against coastal state comply with regulations on pollution, navigation, collision, etc
The difference between innocent passage and transit passage is that transit passage shall not be impeded shall not be suspended.
Note: if there is a strait between the mainland and its island and
also, there is also a high sea/EEZ route seaward of that island then vessels passing through that strait will be under innocent passage regime and not transit passage.