Okay the first, and so far only, female British Prime Minister was Margaret Thatcher between 1979-1990. Her husband was Dennis Thatcher (d2003). I've just been looking on Wikipedia and it doesn't say anything about his role as the first husband of a PM but it does say that he was chairman of the Atlas Preservative Co, vice-chairman of Attwoods plc from 1983 to January 1994 and that he was a director of Quinton Hazell plc from 1968 to 1998, and a consultant to Amec plc and CSX Corp.
I also don't think that there is a term such as the First Lady in the UK. At the moment Sarah Brown is just the Prime Minister's wife and I think that is how the spouse is referred to.
And yes Cherie Blair is a barrister (posh wig and gown and all that) and she did continue working in that position whilst Blair was Pm but she did also do a lot of charity work with Barnardos, Refuge and campaigned for the rights of female prisoners.
here's a link for an article about HER legacy
The president's spouse is a private citizen with no official rank, and thus is properly addressed, in writing and in person, as Mrs. Washington (with neither her nor her husband's given name; she would be the Mrs. Washington, with no danger of being mistaken for Mrs. Chuck Washington).
However, courtesy accords precedence to her, or to another lady serving as the president's hostess. This was referred to, in the era of more complicated and more rigorously observed precedence systems, as her being "the first lady of the land." Hence the title.
And now to the husband. If anything is sillier than "first lady," it is "first husband" (unless this is necessary to distinguish him from a marital successor also on the scene). He would be the host, and addressed simply by his name and "Mr." or another honorific he held, such as general or governor.
Perhaps this is the place to say once again that American protocol dictates that only one person at a time can hold the title of president of the United States. Former presidents should never be so addressed, although they have even taken to calling one another that. Miss Manners would have thought that having reached that position would surely have cured anyone of status anxiety.