I am sorry but this isn't malicious prosecution. This is just the pursuit of a weak case. That happens all the time in particukar with sexual offences in which apparently the view is taken that absence of evidence is just evidence of a suspect's deviousness.
To make out malicious prosecution you have to show some for of bad faith. There have been recent challenges to this under the human rights act which have upheld the position.
If the matter is in court you can seek a defence costs order but it will be only up to legal aid rates.
Even a private prosecutor must bear that risk.
I'm very sorry but the test for these actions is very high.
Can I clarify anything for you?