You are right that the British economy will benefit, hence the muted response.
However, even if the UK government were fervently against it there was nothing they could do. This was a devolved matter to the Scottish Executive.
From the other stories about this, it sure sounds like the British government has been working on this oil deal for years and that the release of this mass murderer was a condition for the deal. The ultimate decision may have been up to the Scottish leader, but it looks bad for all Brits. I’m betting this guy is still alive in ten years and that the 3 months to live thing was a lie to “sell” the release to the public. It’s all about $$$$$$$$$$$$$$$$.