Break in trading and VAT

A client has had a break in trading where they have not made any sales for a couple of quarters.  They intend to start trading again soon and certainly haven't ceased to trade.

HMRC have written to them to say they will be deregistering them, can we do anything about it (you can register on the basis you will make taxable supplies 'at some point', does the same not apply to keeping the registration open?)

The HMRC guidance has 'you cease to make taxable supplies' as a compulsory deregistation case, but they haven't ceased, they've paused!