Right so my friend is leaving India to south America with a stop over in heathrow and Miami with American airlines for £650 next month.

Now if I want to join his flight but get on at heathrow the flight costs £810. Both prices were checked on the same day.

So I'm wondering could I book the flight from Mumbai and just check in at heathrow. Assuming I check in online online, I would already have a seat reserved and a boarding pass for the other 2 flights so it shouldn't be a problem. Would I still be able to check my baggage in at heathrow as well?

Any help will be great, also let me know if it needs further clarifying.