Them being a British public company has zero impact on their level of propriety or dignity or whatever.
Err, yes it does. As a listed company they have certain legal obligations to maintain their listing. If they were to defy this law by taking cash from US citizens then my belief is that the FSA (equivalent of your SEC) would be forced to act resulting in the suspension of their shares and for the stockholders even worse disaster as they then could not sell.
You can't list in the UK and at the same time deliberately break foreign laws (however unjust or however much they are in contravention of the WTO).
In terms of the European and Asian market the respectability of a listing is important but for the shareholders it is essential. They need to stay inside the law, come onshore to the UK, expand outside the US then kick ass at the WTO to get back into the US. It's their only option.