It's reliable
My small company has used Xero for several years now. It is usually reliable, so we're not going to squeal foul because of one outage. As for the VAT you have a whole month to prepare the VAT returns and submit payment to HMRC. If you leave this to the last moment you are asking for trouble. Anyone met Murphy with his JCB digging next to your data line?
Xero does need to look at its major outage procedures and how customers are kept up to date. Lessons to be learned.