use something along the lines of the below sample letter, or write you own:

"Prime Minister,

Your inaction over the Israeli bombings in Gaza and Lebanon is shameful.

The cynical way in which the government is organising the evacuation of
British citizens from Lebanon in order to allow Israel to kill more
Lebanese
with impunity again illustrates your view that Arab lives, whether in
Palestine or elsewhere are worth less than other lives.

You are aiding and abetting war crimes and if you continue to stand by
while
this carnage continues this will be your legacy when you leave office. It
is
simply cowardice to parrot the mantra of Bush and Olmert that Syria and
Iran
are to blame. It is Israel's US- made weaponry which is destroying Gaza
and
the Lebanon and killing hundreds of people.

As British citizens we do not want it to be our legacy, please demand that
Israel ends its attacks on Lebanon and Palestine."