Heathrow overnight layover - transit visa? [duplicate]

Short Answer:

Flying from Canada to LHR to India, your stay at LHR is only a few hours. You can stay airside, not pass through UK Immigration, and transit without a visa.

Flying from India to LHR to Canada, you'll be in the UK overnight. Because the Terminal closes at night, you'll need to pass through UK Immigration to landside when you arrive, spend the night somewhere else, and reenter the Terminal the next day for your flight to Canada. To exit the airport and enter the UK for this overnight, you will need either a Visitor in Transit visa, or you may approach UK Immigration upon arrival from India and seek to transfer without visa, i.e., without having previously secured a visa.

Long Answer:

The previously-cited SE thread contains this govt.uk link which discusses transit and the need for visas. The govt.uk page contains this text:

You might need a visa to pass through the UK in transit (on your way to another country).

Check if you need one before you apply.

To get a transit visa you must prove that:

• you’ll be in transit to another country, with enough funds and the intention to travel on
• you can enter that country
• the only purpose of your visit to the UK is transit

You do not need a transit visa if you:

• are from the European Economic area (EEA) or Switzerland
• have an EEA family permit
• have a Home Office travel document, for example you’re a refugee or stateless person
• have a Standard Visitor visa
• have a Marriage Visitor visa

Following the "check if you need one link" takes you to a question wizard. Answering the question about trip purpose (which I assume is "transit") eventually generates this text:

You’ll need a visa to pass through the UK in transit

You should apply for a Visitor in Transit visa if you arrive on a flight and will pass through immigration control before you leave the UK.

You don’t need to apply for a Visitor in Transit visa if you already have a Marriage Visitor or Standard Visitor visa.

The govt.uk page goes on to discuss an exception, a method to enter the UK for transit purposes without having previously obtained a UK visa:

Transiting without a visa

You might be eligible for ‘transit without visa’ if:

• you arrive and depart by air
• have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive
• have the right documents for your destination (eg a visa for that country)
• One of the following must also apply:

you’re travelling to (or on part of a reasonable journey to) Australia, Canada, New Zealand or the USA and have a valid visa for that country

...

You won’t be able to transit without a visa if a Border Force officer decides you don’t qualify under the immigration rules. You can apply for a transit visa before you travel if you’re unsure whether you qualify for transiting without a visa.

Because your flight to Canada leaves before midnight on the next calendar day after your arrival, and you have a confirmed onward ticket, and the correct paperwork for entry to Canada, UK Immigration should grant you leave to transit without a visa.