There wasn't any need to guard the Ecuadorian embassy, now or then. He couldn't have travelled out of the country without being arrested at the port or airport, and could have been picked up easily if he'd stayed at a house or hotel anywhere inside the UK.
People who are far less well known are routinely arrested by the police for failure to turn up in court. There was no need for this expensive 24 hour guard.
I think they fear that he has well-connected and organized supporters, something most other people fleeing don't have. And he has the benefit of time: he doesn't have to move until everything is perfect, because there is no way of catching up to him where is right now.
If you don't know when your target is going to move, how do you stop him from leaving UK waters before you even realize he isn't there anymore? Monitor every single boat moving on the Thames, 24/7? Search every ship leaving your jurisdiction?
Getting out probably isn't easy, but I would be surprised if there weren't quite a few people/organizations capable of doing it.
People who are far less well known are routinely arrested by the police for failure to turn up in court. There was no need for this expensive 24 hour guard.