Well, it seems she's been cautioned for some offence, anyway: http://www.thisisbristol.co.uk/news/Bristol-MP-gets-police-caution-putting-election-results-Twitter/article-2795724-detail/article.html I wonder what provision the police actually cited.
On the off chance anyone needs the wording…
66A Prohibition on publication of exit polls
(1) 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, or
(b) any forecast as to the result of the election which is (or might reasonably be taken to be) based on information so given.
(2) This section applies to–
(a) any parliamentary election; and
(b) any local government election in England or Wales.
(3) If a person acts in contravention of subsection (1) above, he shall be liable on summary conviction to a fine not exceeding level 5 on the standard scale or to imprisonment for a term not exceeding six months.
(4) In this section–
“forecast” includes estimate;
“publish” means make available to the public at large, or any section of the public, in whatever form and by whatever means;
and any reference to the result of an election is a reference to the result of the election either as a whole or so far as any particular candidate or candidates at the election is or are concerned.