why does time photo was taken changes when iPhone time zone is changed

I set my time zone to London when in London. I was in the GMT+1 (BST ) time zone.

I took hundreds of photos and many videos. All photos and videos had London time in the exif data.

I changed my phone to Cupertino time zone upon returning home to California. GMT-7 (PDT).

This changed the time on all of my photos to 8 hours earlier than they were actually taken.

Changing time zone on iPhone back to London changes time photo was taken to the correct time.

Why is this happening? How can I lock in the time each photo was taken in it's time zone without it changing when I change the iPhone time zone?