Generally, a 'reasonable excuse' is when some unforeseeable or unusual event beyond somebody's control has prevented that person from filing the return on time. For example:
a failure in the HMRC computer system,a serious illness, disability or serious mental health condition has made the taxpayer incapable of filing the tax return etc.
Chas Roy-Chowdhury, May 2013. This paper provides a high-level review of the first 40 years of VAT in the UK, its context in the European Union and its future prospects. We see in the paper the general trend towards governments using VAT to a much greater extent as a significant revenue raiser and the increasing pressures to broaden the VAT base by reducing exemptions and zero rates.