Most of the visa type depends if you are going through the UK border control or not once you arrive in the UK. Even airlines will help you if you need to go through border control. They will tell you if so.

Direct Airside Transit Visa (DATV):

DATV is for the people who are from outside the Europe Economy Area (EEA) and Switzerland and they are going to change their flight and transiting through the UK on the way to another country.  If you belong to this category then you won’t be going through the UK border control but you must take your onward flight within 24 hours of your arrival in the UK. 

If you don’t belong to that country or you don’t have the nationality of the country in which you are traveling to then you must have to explain the reason for your visit and provide the details of your itinerary and accommodation along with a Confirmed flight booking. You must take all your documents with you while traveling to the UK or through the UK.

Visitor in Transit Visa: 

Visitor in Transit visa is for the people who are from outside the Europe Economy Area (EEA) and Switzerland and they are going to change their flight from the UK on their way to another country. If you belong to this category then you will be going through the UK border control that is checking your visa, passport, luggage. In this visa type, you have to leave the UK country within 48 hours and in this period you won’t be allowed to work or study in the country.

If you are traveling to channel islands, isle of man or Ireland through the UK then you may need to apply for a UK visitor visa but if you have a valid visa for channel island than you do not need a visitor visa or even if you have a valid Irish biometric marked visa.

Standard Visitor Visa:

Standard visitor visa is for the people who will be going to the UK for holidays, for business purposes, events or medical treatments. A standard visitor visa has replaced all the visitor visas in the UK, for example, Family visitor visas, child visitor visa, general visitor visas, and many others. Your visa can last for up to 2.5 years or 10 years is a validation of a visa, you can stay there at least for 6 months maximum.

• Original passport (old and new)
• Other valid travel identification.
• Proof of green card, residence permit and valid visa of the country that you are traveling to.

The documentation you need for a standard/transit visa.

• Original passport (old and new)
• Other valid travel identification
• Blank pages in the passport for visa and your passport should be valid for 6 months when you enter the UK.
• A translated certificate that is not in English or Welsh language.
• Financial documents to show you have a sufficient amount to travel. Like bank statements, payment slip.
• Income Tax Return

The documentation provided by the sponsorer

• Passport copy of the sponsor
• Sponsor letter
• Bank statement, Payment slip
• Income Tax Returns
• Residential proof

Documentation for employment

• Original passport (old and new)
• Other valid travel identification
• Blank pages in the passport for visa and your passport should be valid until your whole trip.
• A translated certificate that is not in English or Welsh language.
• Financial documents to show you have a sufficient amount to travel. Like bank statement, payment slip
• Income Tax Returns
• Letter from your company on the letterhead with your role, salary, years of employment.

Documentation for student

• Original passport (old and new)
• Other valid travel identification
• Blank pages in the passport for visa and your passport should be valid until your whole trip.
• A translated certificate that is not in English or Welsh language.
• Financial documents to show you have a sufficient amount to travel. Like bank statement, payment slip
• Letter from your institute on the letterhead confirming your admission and leave of absence…..

Documentation for self-employed

• Original passport (old and new)
• Other valid travel identification
• Blank pages in the passport for visa and your passport should be valid until your whole trip.
• A translated certificate that is not in English or Welsh language.
• Financial documents to show you have a sufficient amount to travel. Like bank statement, payment slip
• Income Tax Returns
• Business registration documents that can confirm the owner of the business and the date, when it was started trading.

Documents for under 18s

• Original passport (old and new)
• Other valid travel identification
• Blank pages in the passport for visa and your passport should be valid until your whole trip.
• A translated certificate that is not in English or Welsh language.
• Birth certificate or adaption letter if the child is adopted.
• If traveling without parents or guardians then a signed letter from them confirming the details about the person who is accompanying you along with the detail who will take charge of a child in the UK.
• A copy of a child’s parents or legal guardian’s biographical page of passport including their signs and passport number

People who Transit through UK are mainly Visiting to Schengen Area, Apply Schengen TouristVisa with Fly For Holidays

Applying from outside the UK then you can apply online and fill the form but you must have your bio metric interview that is photograph and fingerprints at the visa center. Countries like Afghanistan have to apply through Pakistan, Andorra has to apply through Spain, Bhutan from India and many others cannot fill the online form.