Cameron could also have vetoed Brexit, but didn't. Remember that the government of the day opposed Brexit but still honored the referendum result. There's similarly no reason to think that the UK government would have vetoed Scottish independence if the previous referendum had gone the other way [1][2]. There's a distinct possibility of a future referendum having a different result.

[1] https://www.legislation.gov.uk/uksi/2013/242/pdfs/uksi_20130...

[2] https://en.wikipedia.org/wiki/Edinburgh_Agreement_(2012)