They had the parish priest in a while ago as well which was good - we're Reformed Evangelicals ourselves and I suspect that some of the older crustier church members would like me to tell the kids that all Catholics have horns and tails.
Our primary is a church school (C of E) but is careful to ensure that all Christian denominations are invited to lead the shared worship, so we often have visits from the RC priest, the Methodist minister and free church representatives as well as the local Anglican vicar. Speakers are also invited from other major religions and the school organizes visits to mosques etc. so that the children learn about other faiths, as is indeed required under the National Curriculum.
We also have representatives of charities who visit from time to time to give a talk about their organization and its aims, followed by a fundraising activity for that charity (e.g. a school in Africa or an animal sanctuary). I'm not sure about political lobbying organizations as such, but as push-pull mum says, provided it's done properly I think it would be very beneficial. Perhaps parents could br advised in advance that such visits are taking place, and invited to attend as well if they have any concerns.