I think some us are questioning the wisdom of such a “borrowing”(as you put it) in a time of unprecedented terror threat when a strong hand may be needed. The war making powers enumerated in our constitution have been an organic part of our government since its birth and we have a better sense of the check and balances governing our selves.
To suddenly thrust that kind of power onto a legislative body (Lords and Commons) which is not known for nimble response to complex social issues, is to invite socio-political paralysis at a time when flexible police/military responses may be needed.
Flexible police and military responses have nothing to do with a formal declaration of war though. We managed to retake the Falkland Islands without a formal declaration of war.
In reality it would always be difficult for a Prime Minister to declare a war without the support of Parliament. All this move really does is formalise the precedent set by Tony Blair’s debate and formal vote in the House of Commons prior to the Iraq invasion.