* PM Cameron says strikes needed to tackle Islamic State * Parliamentary motion expected to win cross-party support * PM looks to banish memory of 2013 Syria parliamentary loss (Updates after start of debate, adds Cameron, lawmaker quotes) By Andrew Osborn and William James LONDON, Sept 26 (Reuters) - Prime Minister David Cameron urged lawmakers on Friday to vote in favour of Britain joining U.S.-led air strikes against Islamic State militants in Iraq, hours ahead of a crunch parliamentary vote he is expected to win. ...