Despite growing public disquiet British Prime Minister Gordon Brown has vowed not to walk away from Afghanistan.It follows a spate of British troop deaths this week, including five soldiers killed on Tuesday by an Afghan policeman. In London, Brown said the military mission was vital to prevent attacks against Britain being launched from the region. ‘‘I reject any suggestions that our efforts to tackle the terrorist threat abroad have distracted us from mounting the strongest possible defence against the threat at home. At present the biggest domestic threat continues to come from the mountains of Pakistan and Afghanistan, so it is right that our first line of defence is there,’‘ Brown said. Support for the conflict, however, appears to be wavering. One recent poll showed more than a third of the British public want UK forces pulled out of Afghanistan immediately.