Accountants....
Most reputable accountant agencies are quite well versed in UK tax law / Intl implications. The initial inquiry is often free - to determine your position and their interpretation / avenues to pursue. They will not of course give free advice, but will provide an honest opinion if they can assist. THey will also agrue with the IRS etc your case.
...personally, as an ex- foreigner, it will cost a little - but IMHO WELL WORTH it in peace of mind, and Factual advice.