To become a neurologist and presuming you mean the usual definition of a doctor practising neurology, then you would need a degree in medicine before undertaking the speciality training.
There's information on the NHS careers websites about diferrent specialities etc.
As you are doing an undergraduate degree (again presuming in the UK with intention of working in the UK afterwards), you have the option once you graduate of either doing undergraduate medicine (5-6 years) as a second degree, or graduate entry medicine (4 years). GEM is extremely competitive and far more so than the undergraduate degree however, so I would do some research on this.