Corrected entry: During their assessment of the security in operation at the Syrian bank they intend to rob, Mickey and his team discuss in some detail the regular patrols of the safety deposit box vaults by guards leading attack dogs. These guards are conveniently forgotten about during the heist, even though there has been a power failure knocking out all but one of their electronic safeguards. You'd think there would be increased and more thorough patrols by the guards at that time - instead they just disappear.
Correction: The guards are shown patrolling with their dogs. Mickey and his team are in a vault to which the guards have no access. If they did continue their patrol inside the vault, they would set off alarms triggered by the pressure pads in the floor.
Corrected entry: If MI5 and the Serious Crime Squad had information to the effect that a terrorist suspect had stashed valuables in a safe deposit bank in a vault in London it would take them about ten minutes to obtain a confiscation order signed by a Supreme Court judge. The bank wouldn't even know about it until the heavy squad turned up and if they claimed they couldn't open the deposit box in question the locks would simply be drilled out on the spot. No risk and no unexpected surprises from the criminals you have hired to pull off an extremely hazardous operation (as happens here) - the whole thing would be neat, clean, and above all legal.
Correction: This is explicitly dealt with in the episode. The female MI6 agent explains that if the Home Office were to raid the bank it would create a hell of a row with the Syrian government, whereas having a bunch of hapless crooks arrested during a robbery would mean nothing, as HM government would be able to deny any involvement.
Correction: Watch the episode again. The guards are shown patrolling, but Mickey and his gang are behind a huge, locked vault door and the guards do not have access. One guard is shown looking at the door and is satisfied it is closed and secure. The guards have every reason to believe that the pressure pads inside the vault provide sufficient security during the power cut as they are not affected.