I have been looking into this matter as the police's actions seem quite unusual.
I have spoken to a forensic toxicologist regarding this issue and there is in fact no legal limit for urine in respect of the new drug driving laws. They have also advised that it would be incredibly difficult to try and translate the result of a urine test to an equivalent of blood as there are a significant number of variables to be taken into account.
The police are however still able to charge you with driving whilst unfit through drugs. When the legislation was updated it remained possible to charge someone with the offence, provided that you can still demonstrate that they had drugs in their system and were impaired as a result of this at the time of driving. It may well be the case that if all of the adverse effects had ceased by the time you were driving then the police would struggle to prove that you were impaired.
Because of the unusual nature of the case I would strongly advise obtaining representation if you are minded to fight this. If you are interested in representation you are welcome to call my department on 0333 999 7158.