Notice of return for cancelled registration and closed business

A proprietor cancelled their voluntary GST registration without filing a return for AY 2018-19 and has since started a new business in another state. They've now received a notice for the old registration, demanding a return filing within 15 days or facing assessment under Section 63, with a potential £10k penalty. The advice suggests filing the return with the penalty to avoid best judgement assessment and potential complications with the current business, which is linked by the same PAN.