Yes, old fruit, you will need to pass through UK passport control before being emptied into the terminal. And yes, you will need to check in at the Delta ticket counter before going up to your gate. The agents there must give you a boarding pass before you will be allowed to pass through security.

And why, pray, are you going to London just for the day? Are you insane? If so, you will fit in well with the notably eccentric Brits.

Cordially,