Conscience, http://www.conscienceonline.org.uk/ is an organisation promoting the idea of taxes for peace not war. It campaigns for a progressive increase in the amount of UK tax spent on peacebuilding, and a corresponding decrease in the amount spent on war and preparation for war. It also campaigns for the legal right of those with a conscientious objection to war to have the entire military part of their taxes spent on peacebuilding. This latter idea is known, rather misleadingly, as the Peace Tax.
There seem to me three arguments against the Peace Tax. Firstly, there is the slippery slope argument. To allow a legal right for those with a deep personal or religious objection to some aspect of government expenditure is to open the door to any number of single-interest enthusiasts, most obviously the anti-abortionist. Medical staff are already able to decline to participate in abortion procedures on grounds of conscience. If the right of conscientious objection is granted in respect of taxes going towards military expenditure, a Pro-Lifer could reasonably argue for the right on grounds of conscience to disallow any of their tax payments going to fund abortions on the National Health, insisting instead the money going on life-sustaining medical activity.
Secondly, in the UK donating through the Gift Aid scheme means charities can claim an extra 25p for every £1 given, so if it is open to a peace campaigner to donate to the organisation of her choice, which then gets back public money which would otherwise go on defence. In effect, there is already a means for a tax-payer to divert public funds to a peace charity and accordingly away from objectionable military expenditure. No doubt Conscience itself uses the Gift Aid scheme, which would be slightly ironic.
Thirdly, administering a Peace Tax would be burdensome for the government and of little benefit to tax-payers as a whole.
There is an excellent video on the history of Quaker war tax resistance https://www.youtube.com/watch?v=W658HYZGgPY. The speaker makes the point that life these days is too complicated to permit of simplistic measures like the Peace Tax.
Conscience are sponsoring a Peace Tax Bill in Parliament. It is a good way of drawing attention to the organisation's correct and commendable principle that spending money on peace-building rather than on defence is much better value for money, if one may put the issue just in economic terms. However, I expect the Bill itself will make little progress.