If there’s a good reason to scrap the Monarchy in Britain, it has nothing to do with the cost or symbolic arguments that often are raised.
The most significant constitutional disfunction which Monarchy gives to Britain is the royal prerogative powers which are, in practice, delegated to the Government of the day. Under the royal prerogative powers, a government can declare war and deploy armed forces without the backing or consent of Parliament.
It’s therefore a pretty good idea that the House of Lords Constitution Committee is launching an investigation into the war-making powers under the Royal Perogative. They’re inviting evidence.