Those economists are not accounting for the nuts behaviour of the British Public.
The prospect of things being closed for a day combined with an extra bank holiday weekend is bound to result in some people panic buying. Just so they definitely have things in to celebrate the Queen. The wise will avoid the shops this weekend! (hint: they are open again on Tuesday and you can still order things online...)
A lot of the pubs around here are either staying open or are opening after the funeral has finished.