This is a tough call to make, I can see justifications for her remaining in the NLP (and being disgruntled with Thorneycroft's leadership) but also for her defected to the Tories. So I'm not entirely sure, although I suppose it is something I really need to decide soon! With the NLP's smaller pool of major politicians she would likely become a significant figure in the party much earlier than she did in the Tories.
How about if NLP and Tories lose in this election and cannot enter government, they shall hold talks about a merger in order to keep fighting the left better. Merge the parties, but allow factions to exist and to be voted for. It is around these times anyway that conservative parties start more opposing Keynesians and embrace free markets and smaller states (more or less...)
The tragedy is that while they Conservaties (Thatcher, Reagan) run in elections for policies that were supposed to limit the state, the state still grew ever greater under their reigns...
I'm open for a merger as long the Tories accept sensible old school Austrian policies.

And I might make compromises that will make me cry a bit, but whatever is necessary to combat the left must be done.
(this allows a more historical course, with Thatcher entering the big play)