"It was a thoughtless thing to do," she said, and no doubt it was but so is answering the phone while driving or walking out of a shop without paying for something you have taken and none of us would expect leniency for that.
The Bracknell Blog goes into great detail about what McCarthy has done and why it is wrong. Section 66A of the 2000 Representation of the People's Act says:
No person shall, in the case of an election to which this section applies, publish before the poll is closed: (a) any statement relating to the way in which voters have voted at the election where that statement is (or might reasonably be taken to be) based on information given by voters after they have voted...By revealing how a sample of 300 voters have cost their ballot paper in her constituency, she has clearly broken that regulation. The punishment is a fine of up to £5,000 or a maximum of six months in prison. Send her down.