I would sell it and probably buy another - despite not knowing how reliable the next car would be. This is because if I knew I had to pay the ULEZ that would definitely stop me from using it most of the time, as it has done you. Or sell it and not buy another car but pay for a taxi as and when needed. This would still feel painful to me, so I'd need to keep thinking how much I'd saved by not paying tax, insurance, MOT, repairs etc and see how many taxi fares that covered.