Actually, it really
was an unconstitutional action;
'As noted in the Cabinet Manual, the current state of play is that Parliament has been consulted on all significant military deployments since Tony Blair sought approval for the UK’s military intervention in Iraq in 2003 - setting quite a precedent in constitutional terms.
So why is the Government so keen to disown and disarm the convention now? From a constitutional perspective, it’s important to note that the the convention allows the government a great deal of flexibility, providing exemptions from parliamentary consultation for emergency situations or where the need for urgent or secret action precludes advance parliamentary consultation. Given the grim seven year history of the conflict, however, it seems difficult to argue that the action being considered is strategically urgent. Nor, in the context of Trump’s prematurely ejaculated tweets, could one argue that any deployments might be secret.'
https://www.huffingtonpost.co.uk/entry/theresa-may-syria_uk_5ad0c787e4b016a07e9beb19