As far as HMRC are concerned, there are two types of company option scheme, approved or unapproved. Since your scheme was probably designed for the US tax regime, it will almost certainly be an unapproved one as far as UK tax is concerned and that's why your gain will be taxed through the payroll as you will read here. There is really no way around that if you want the cash.
In some situations it is possible to swap the options the employee has in the company being taken over for options in the company doing the takeover. You'd have to discuss that with your share option plan manager.
I hope this helps but let me know if you have any further questions.