Accordingly, do we need transit visa for British Airways?
Transit visas are available if you are staying for less than 144 hours. If you have a British passport, the GOV.UK website shows what types of are available when travelling to or through China.
Also, do I need a transit visa if I don't leave the airport? Many countries that require visas do not require them if you're simply transiting through the airport. You bypass customs and immigration, so you never get stamped into the country, but you can't leave the airport during your layover. If you want to leave the airport in one of these places, you'll have to get a visa.
Simply so, do I need transit visa if I travel via London?
At London (Heathrow), there's the Direct Airside Transit Visa (DATV). “If you're transiting London you need DATV. Similarly, if you're travelling to the US and the flight is routed over Canada, you need a Canadian transit visa. For most of other countries Indian nationals don't need airport transit visa.
What happens if you don't have a transit visa?
When you check in for your an international flight, the airline will check your passport and visa. If your documents don't meet the requirements (according to a detailed list of immigration requirements by country, in a book published annually by the IATA) then you will simply be denied boarding.