Inflation!
So what *is* the downside?
An inflation rate of 40% a year is a massive drag on growth, no matter how predictable it is. Plus, you've just eliminated *the* major demand for £s on a day to day basis: the necessity of collecting enough of them to pay your taxes. In your scenario I'd expect the private sector to go underground as much as possible & move to some other medium of exchange.
What might be sensible is to stop bothering with the ludicrous idea that the government should borrow money at interest in order to fund a short term deficit: there's a decent case to be made that deficit spending *should* simply be printed.