Upvote:1
I have to change both airline and airport in London
Airport changes require you to go through immigration control. In fact, you will spend significant time travelling around London.
So yes, you will need a transit Visa.
Check your details at https://www.gov.uk/check-uk-visa
I don't think the Italian Residence card makes much of a different here. You are a EU resident but not an EU citizen and you will still need to travel with your Tunisian passport.
FYI: this is not a great itinerary. Changing airports in London is inconvenient, time consuming and expensive.