David Cantrell wrote:

The other problem is that elected Mayors are not seen as a stepping stone
to
higher things because it's hard to know if one can return to the Commons,
so
the ambitious are not drawn to the post

Johnson isn't ambitious?

He has ambitions but he was also somewhat drafted rather than initially
seeking it - and there were signs his Commons career was stalling back in
2007. Furthermore many Conservatives seriously doubted Livingstone could be
defeated in 2008. Runing Boris for the Mayoralty was a gamble that paid off
for the party at the tiny cost of subsequently losing him from the Commons
but for Boris it's removed him from the main centre political attention
without a clear route for returning.

People talk of Boris as a potential future Conservative leader but they
forget that it's difficult to make a successful return to the Commons and
obtain the top prizes, and especially so in the short run. The last major
party leader to have had an interrupted parliamentary career was Michael
Foot (who was out between 1955 and 1960) and he obtain the Labour leadership
20 years after he returned. The last Conservative leader was Sir Alec
Douglas-Home - leaving aside his transfer from the Lords to the Commons he
had lost his seat in the 1945 election before regaining it in 1950, but
again that was over 13 years before he obtained the leadership (and at
several stages he had astounding luck in his career). The last Liberal
leader was Sir Herbert Samuel who lost his seat in the 1918 election then
had a career outside party politics before returning to the Liberal battle
and the Commons in 1929 and becoming the leader of the main party in the
1931 splits. The most recent other case is Roy Jenkins coming back from
Europe and forming the SDP but their failure is a deterrant to anyone else
trying the route of a new party.

And of course there are the failures - both Michael Portillo and Malcolm
Rifkind fell in the 1997 general election and so were absent from the
subsequent leadership election. Each eventually got back to the Commons but
by the time the leadership came up they found their original political bases
had moved on. Conservative MPs still retain a lot of power in the leadership
election and it's questionable whether a returned Johnson would have
sufficient support to make it to the final two and thus go to the full
membership.

If Boris is re-elected in 2012 then his term won't expire until 2016 and to
get back into the Commons he would either have to wait until a general
election in 2020 (thanks to the fixed terms legislation) or win a
by-election. But if the Conservatives are still in power after 2015, which
many seem to expect, then being the government candidate in a by-election is
a risky strategy, especially if it seems the candidate wants to go straight
to the top. However if he waits until 2020 then it's likely that by then
Cameron will have already stepped down and a new leader will have been
elected so Boris would have missed his chance. The only other possibility is
if he could contest the leadership as a non-MP - I forget if the rules are
explicit on this point, but it's a very risky strategy.

The main Westminster style country where this happens a lot is Canada, where
party leaderships at both federal and provincial/territorial level are often
contested by non-MPs, with former MPs, MPs from the other level, Mayors of
big cities, private businessmen, union leaders and others often standing.
There is also a partially observed covention in Canada that if a leader
doesn't have a seat then another MP will resign and the leader will stand in
the by-election without opposition from the other parties. (It's not always
observed and can sometimes go wrong - John Tory in Ontario is the most
prominent recent case.) Some leaders don't have a by-election and instead
fight a seat in the main election, even if they've already been appointed
Prime Minister/Premier - John Turner in 1984 is the most prominent federal
case.

Over in Australia this practice may take off if a current experiment works -
in the state of Queensland the Liberal National Party is going into next
year's election led by Campbell Newman, who has just finished as Mayor of
Brisbane and is leading from outside the state legislature. Brisbane is
Australia's largest local authority (most of what we think of as the big
Australian cities are collections of multiple local authorities - Brisbane
is the only case of a super city authority taking effect) and the Mayor is
directly elected, so this could be one to watch for Boris's chances.