Best avoided or find a longer connection.
You are responsible for transportation between airports ... if you have bags, you will have to collect them at LGW and take them to LHR and check-in again ...
https://www.flyertalk.com/forum/brit...onnection.html and
https://www.flyertalk.com/forum/brit...ial-guide.html
BA will re-book you ... but there are too many variables outside your control to make this a stress-free option ...