Returning home after date stated in landing card

[Haveasafetrip]

Hello,
This is my post topic and I have a doubt about return date.
I arrived in the UK and I am a non-visa national, so I received a 6 month stamp.
I stated that I would just stay some days but I decided to extend my visit for some more time because I could postpone some obligations I had at home and I can have a bit more of holiday. I am staying at my boyfriend's house and I included his address in my landing card.

Will I have any problem? If i don't leave on the date stated on my landing card will I be an illegal immigrant even tough i have a 6 month leave to stay (stamp) on my passport?

Do I need to contact immigration to inform this change?

[MPH80]

No, No and No.