The mega-value reader who first alerted me to Boingo's UK woes follows up with this, also from the company's customer service department - apparently everything works as long as you lie about your country of residence:
"Thank you for contacting Boingo Wireless. For UK residents, during account
creation, if one selects Ireland in the country pull down, this will circumvent
the UK restriction. However please be advised of premium pricing on certain
UK locations. Refer to the location directory on our home page for details."