On the face of it, the employee was a bit silly about how he went about things. He should really have asked Harrods to erase the personal data for him (say in his presence), at which point he hands it back. As a data subject, he has the right for it to be deleted unless they can show a valid business reason for keeping it there, which sounds unlikely.
Given they didn't know he had the laptop, that's a pretty poor state of affairs for Harrods as well. The shop was certainly being somewhat over zealous and actually acting wrongly. They suspected a criminal offence was being committed, which means they should have secured the laptop and contacted the police. That ensures the evidential chain. If they suspected something that wasn't criminal, they could contact Harrods. After all, the police investigate criminal matters and pass files to the DPP, not Harrods.
You can quite easily ask a shop to perform actions on a laptop that isn't yours (say a company one). There's nothing wrong with that at all. After all, businesses need work done and sometimes local shops can be as good a route as anyone.
The theft charge simply seems to be an extrapolation of the attempt to access, with the assumption being he wanted to retain use of the laptop and not just delete the personal data. That's one hell of an extrapolation unless there was additional evidence showing motivation. Having just been made redundant isn't evidence of this at all. So, no party really comes out of this well. The employee was somewhat stupid in his actions, Harrods have shown a lamentable grasp on who has their kit and the prosecutor seems to be into knee jerk overreactions.
Hence, we end up wasting god knows how much time all round (police, DPP, courts etc.) for something really trivial. Guess it's easier than persuing real, personal crime though........