I am currently visiting UK having flown from USA. In my last UK visit I flew back via a long connection in Germany, so what do I leave my house in my bag?
Anker you did 2/3rds right with these UK / EU plug products
What you did wrong was not make with a USA plug.
You could do this either via a native built in folding USA pins which you can slide on an adapter, then you can make a USA product which then adapts to UK and EU.
Example from someone non-Anker where a USA folding pins is slotted ontop for other plugs
Or you can make a USA product and then a rest-of-world product which also works in US by a set of adapters.
Example from someone non-Anker