Per the FlightRadar24 logs, it looks like only about 45min was wasted over Prestwick, not 2hrs. First approach was around 18:06, and they're breaking off to head for Edinburgh by about 18:51.

If there's considered to be a mistake here though, I'm guessing it's going to be spending too long before committing to the initial diversion.

Without knowing the weather they were seeing at the time, seems hard to say if they should have gone for a closer 2nd alternate than Manchester.