You make a reasonable point OP. Why spend untold millions to go to Mars when there enough problems down here to fix? Ie the environment, reconstructing Syria, lifting people out of poverty here and in the 3rd world etc etc..
I personally have mixed feelings for example, I wouldn't want to try want to explain to the many disabled people now waiting for the letter through the door, telling them their benefits are being cut. Just how exciting and beneficial to them, sending Tim Peake to the ISS is, particularly.
But if I had to, I'd start by saying - there's so many remarkable benefits from space programmes, personal computers and solar energy for a start. Space science has given us new materials, better medicines, more efficient ways of providing clean water, and ways to grow food. It's increased our understanding of the human body, giving us innovative ways to protect people from different illnesses. Also the UK has a service-based economy, so striving to be a nation of scientific innovation is important.
Then on the other hand though, what happens when these remarkable breakthroughs in tec' happen? They get patented by big corporations, manufactured by them and sold to us at a high price which excludes the poor and /or the third world from gaining any real benefit from them. In that respect all the incredible discoveries and innovations gained from the world's various space programmes don't seem to effect human behaviour much.
Boring and non-committal of me, but I can't decide really.