This question already has an answer here:
I'm now in the UK with a Standard Tourist Visa (valid for six months until August) for my graduation ceremony and a holiday. I arrived here on the 10th of Feb and planned on leaving on the 10th of March (so I'd stay here for a month). I've written the date of my intended departure date on the immigration (arrival card) and the officer took the card when she interviewed me at the airport and then let me enter. Unfortunately three days after my graduation I caught a cold and it triggered my asthma. I was sick for over a week. Considering this, I'm thinking of extending my stay until end of this month. But I'm worried if this is allowed. Considering that I've already put my departure date in the immigration/arrival card.
If I did extend, should I inform the immigration? And if so, would this be a problem the next time I visit the country?