Welcome to FT

The problem with using a UK plug is that it's not uncommon for the outlets on planes used for domestic/Canada/Caribbean service by U.S. and Canadian carriers to have outlets that only fit North American and Continental European plugs.

The UK plug was a great solution on LH and SN but on UA and DL it often won't fit in the outlet. I've never seen an aircraft outlet that won't accept a type C (ungrounded European) plug, though, and I was just on a DL flight last month where both outlets in my row were too worn out for our American chargers. The Europlug adapter that still lives in my bag saved the day.