Do you need a passport to fly to London England from Cailfornia USA?

Yes, you require a passport to fly to London Ontario from England. Similarly if you fly to London England from Manchester via Hong Kong you will need a passport

+++

Not sure why you'd fly from Manchester to London all of 200 miles away, via Hong Kong! No, you don't need a passport to travel within the UK, BUT the airports demand ID for security, and your passport is usually the most reliable.