Well looking at the flight log it looks like it has indeed landed safely around about 40 Rotherhithe St.
That is where I would go and start making enquiries. It might even be on a rooftop or up in a tree in that vicinity.
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It took you 54% to get as high and as far as you did and it initiated RTH AT 46% so theoretically your remaining power was insufficient to RTH
The biggest mistake in this is your inexperience I'm afraid..... At 25% power it initiated Auto landing you could have cancelled this and flew it closer home as much as the remaining
battery would have allowed.
It acted correctly all the way it was returning to home like it should but due to the distance and height you flew it to it hit the low
battery which must be set to 25% in your parameters.
Also (And I do apologise as I am in no way trying to tell you how to fly) The UK height limit for flying a drone is 400ft.... You can be prosecuted for anything above this.....
Hope this helps.... So my opinion is Pilot Error.....