ICAO licences cover those countries which opt not to have their own bespoke licensing system such as Hong Kong and West Indies, the writtens are a derivative of the CAA CAP 509 exams I believe but they still kick you with negative marking. ICAO don't ISSUE the license they are issued by the state in question but the License is termed an ICAO license. So I understand anyway!
As for whether they are being accepted by airlines in the UK I think it depends on whether they are on good terms with the CAA and therefore whether they are likely to give an endorsement for that license in lieu of having a CAP509 or JAR license.