I don't mean to sound as if UK carriers are not capable of providing good reliable service.
In this particular case, Omni happened to win out.
You must consider that perhaps one of the reasons the contract went outside of the UK is simply that there is a smaller number of carriers and available airplanes in the UK. Those UK carriers quite possibly had their hands full with other contracts. Nothing wrong with that.
There are more available airplanes for use in the US, so it stands to reason that countries with less of their own airline capacity will hire planes from the States and other places.
We've been hired to fly for Japanese companies as well as companies in Kazakhstan and Iceland and many others. Not that the air carriers are substandard in those countries, they just don't have the available airplanes and the available seat capacity. So they hire outside of their country.
Not a big deal. They just want to get the airplanes in to get the job done.