Transit visa in London for travel to Halifax Canada

I am traveling from Delhi to Heathrow and then Gatwick to Halifax next month. I have Canada study permit.
I have booked a single itinerary ticket on different airlines (Virgin and WestJet). I will stay overnight in London. Does Study permit qualify for the exemption and can I transit without visa. I will be changing airports in London.

Check this link:

In most cases I have heard you do not need visa if you are only transiting (esp. with a US visa), however your situation might be different since you are changing airports, and I don’t know if things are different with a Canadian visa.

Since you are passing through UK immigration you will need 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.
This applies to me.
Just completed a journey yesterday through Heathrow and Gatwick with the same conditions as the OP listed.

Our party consisted of 3, 2 of us have US passports, one has an Indian passport with a Greencard.

This was the return leg from Italy to Gatwick, then Heathrow back to the US, but the inbound journey was through Heathrow only.

The key thing the immigration officer wanted to see is why there wasnt a stamp on the inbound trip, I then went on to explain that we never left Heathrow.

He was also impressed that 1) I had the itineraries printed 2) the boarding passes to the States ready 3) the UK website with the requirements.

Anyways the whole thing was over in 5 minutes and we were on our way to Heathrow he has stamped a transit visa for 48 hours on the Indian passport.

I hope I am interpreting it correctly.

I transited successfully without needing any visa.