Before deplaning, you have to make sure you have with you your passport, the filled out arrival card, return ticket and have on standby other supporting documents.
As you deplane, you go straight to the immigration lounge. There’ll be two queues: one for EU/UK passport holders and Non-EU passport holders. Pick your lane.
If you’re wearing a hat and sunglasses, take them off. Once called by the Immigration Officer, hand in your passport, arrival card and return ticket. Answer the Immigration Officer’s questions. Don’t worry. Usually, you’re gonna be asked about the date of your return, how many days are you gonna be staying in the UK, what’s the name of the person you’re gonna be staying with, where do you intend to stay, etc. Some other time, the IO is just being chatty, so don’t get to worked up about it.
After the immigration phase, you’ll need to get to the baggage pick up area to collect your luggage and go your merry way.
Enjoy your stay in the UK.