The forms of photo ID required for internal flights (including the Channel Islands and the Isle of Man) and (in some cases) for flights to the Republic of Ireland vary between airlines. Here are a few examples:
FlyBe (who include the Isle of Man in their destinations) will accept a Citizen Card. They'll also accept an expired passport for up to 2 years after the date of expiry. They'll even accept a pensioner's bus pass. For the full list, go here:
http://www.flybe.com/
Then go to 'Your Flight' > 'FAQs' and select 'What photographic ID will I need when I check-in?', under 'General Information & Policies'.
Ryanair (who fly UK-Ireland plus some UK domestic flights) won't permit you to travel without a
valid passport or a driving licence:
http://www.ryanair.com/site/EN/faqs.php?sect=p id&quest=photoid
Blue Islands (who serve the Channel Islands) have a similar broad policy to that of FlyBe:
http://www.blueislands.com/en/legal/legal.aspx #ID
BA (somewhat surprisingly) state "We do not require to see a passport or photo identification if you are travelling on purely UK domestic flights":
http://www.britishairways.com/travel/askbainte r/public/en_gb?p_search_text=id
Chris