I have had several flights moved from LGW to LHR and each time the 'Important information about your booking" email has stated "We're very sorry to let you know that the following flights on your booking have been cancelled". Even when the new Heathrow flight has the same times and flight number.
The e-mail also goes on to say "If you accept to travel from a different departure point or to a different destination than your original booking, we will not cover you for any expenses for travel between the two points, including car rental and parking". So this seems to be at odds with 8.3 above.