ApolloC, first, you can get your visit visa upon arrivat at the airport. Later this visa will be converted into your work visa, and you will go through the proccess of getting your Residence permit.
Regarding Police Clearance. Different Companies follow bit different rules. Barwa DO need the Police Clearance. So you better either bring it from UK (even not authenticated), and then you can legalize it here. Or do as per Brit's suggestion. It looks like he has done it, so he knows what he is talking about.
aviduser , with all respect to you.. Barwa is a stable and strong company. I don't think they would still hire new employees if things didn't go well for them...

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“You become responsible forever for what you have tamed”. Antoine de Saint-Exupéry