Isn't it the opposite, i.e. share issuance? With lower taxes the government leaves more money in the system (and generally borrows/prints more) which lowers the value of each unit of currency, hence the pound declines.
Keynes policy proposals for the government have an obvious flaw, nobody really follows them unless he gives the government permission to spend (which is obviously a valid move in a depression or during COVID). This is why I think Keynes theories have had more success in montary policy than fiscal policy.