Again, just try to get there 2 (or more) days early & enjoy your days in London while getting over jet lag. You won't be stressed about missing/changed flights or missing luggage & it's so very worth the few extra dollars, in the whole scheme of things, in the long run. Maybe you're a repeat visitor to London, you haven't said, but if you've not been in a while, it's a good time to explore new areas while not feeling rushed. One thing you can be sure of, your flights will change between now & then. It's a jungle out there. Good luck with your decisions.