AFAIAA (and i've searched quite diligently!), there is no Parliamentary rule which precludes a Chancellor from releasing Budget details pre-Budget day. There is, however, a well-established convention that contents of the budget should be withheld until the Budget speech.

So it would appear that, no matter how much the Right-wing Rage-Bait Rags may wish it, Reeves is not guilty of breaking the rules over the 'leaks' (and, if she had, a number of previous Chancellors, somre of them Tories, must have been guilty of exactly the same kind of transgression).

The 'Lying to Parliament' question remains but, again, there have been many MPs, including leading Tories, who have been economical with the truth in the past and got away with it.