I want to do something and want to know if it's legal. And if it's not, is it POSSIBLE? And if it is ILLEGAL and POSSIBLE, then will the airline do anything to me (i.e. charge me, etc.) if I decide to do it?

I buy a ticket on BA:

Toronto - London - Madrid - London - Toronto.

I use the first 2 ticket cupons: Toronto - London - Madrid. I skip the third cupon: Madrid - London. I use the forth cupon: London - Toronto.

1) Is this legal?
2) If not, can I still do it?
3) Will my reservation for the last flight be cancelled if I don't show up for the flight before?
4) Should I do it?