im no expert with plugs but i bought a u.s plug 3rd gen echo dot aswell , i bought a all in one unversal u.s aus , eu travel adaptor ive seen the same adaptors on ebay for 99p mine works just fine just make sure the 2 prongs that go in the adaptor are not upside down .
to ensure this the writting on the adapter is the same way up as the writting as the amazon u.s plug (my u.s plug sits lower than adaptor but works fine) its at your own risk of course ., or you can buy the uk lead on amazon uk for 14 pound ,. but my 99p adaptor works fine with the u.s plug like i said
good luck hope this cures most peoples problem it worked for me