What's the best thing to do with an old diesel car?

I live in the new ultra-low emissions zone, so I’ll have to pay £12.50 a day. What should I do?Every week a Guardian Money reader submits a question, and it’s up to you to help him or her out – a selection of the best answers will appear in next Saturday’s paper.
I have a 15-year-old diesel car in south London. I live in the new ultra-low emission zone, which from 2021 will mean I have to pay £12.50 a day. I could sell it, but it’s probably worth less th