I live in Iran and I am going to take a summer course in Bermuda(for one month). It seems Bermuda does not need a visa and I want to know that getting a UK transit visa will allow me to enter to Bermuda? Also, there are two kind of UK transit visa (Airside and visitor in transit), could I change my airport with the visitor in transit UK visa to catch my onward flight?
The Direct Airside Transit visa will not allow you to pass immigration or change airports. Only some nationalities require such a visa to transit the UK airside (not passing immigration); many others can transit the UK airside without a visa.
If you need to change airports, the Visitor in Transit visa will allow you to pass immigration to get a hotel and/or change airports. Your onward flight must depart within 48 hours after you arrive (the UK does not operate exit immigration controls at airports; your departure is recorded for immigration purposes when your plane departs with you on board).