Departure tax from Heathrow

You don't pay it separately - it is all rolled up into the ticket price. Do you need to have a breakout?

It is the same how ever long you stay after 24 hours.

I just came back. I think it was $39 for the tax plus more for the security fees, so $50+ total.

But the point is that the tax is built into the cost of the ticket. You don't pay it yourself at the airport like we did departing Japan some years ago.