The convention is that HMQ can refuse to give assent, but only in exceptional circumstances (e.g. on presentation of the Gordon Brown President for Life Enabling Act).
She has the power to refuse anything she damn well pleases, but as this would raise constitutional questions regarding royal prerogative/separation of powers, etc it is regarded as being 'form' for her not to do so. If she did, the theory has it, it would inevitably lead to the reduction of the monarch to nothing more than a figurehead.
Only in a case such as the above spurious Act would refusal not create a situation.
However, so skilled have Tony and Gordon been at fouling things up, I have a nagging suspicion that they've actually created a situation in which HMQ could veto a bill, sack the PM, dissolve parliament and end up with her powers enhanced as the result of the subsequent election.
Edit - It's a myth to say we don't have a constitution. We do, it's just not written down. The British constitution is based upon custom, precedent, statute law and its pragmatic (or sometimes not) interpretation.