My knowledge of UK legislation is a bit dim now but when I last operated on the UK register a pilot could not exercise his/her discretion until immediately before it was required, i.e. prior to thr LAST sector, then only two hours to get home.
The third hour was available for matters concerning safety only, for example arriving at destination to find it unusable so having to divert and use some of the third hour etc. The third hour required a written report to the CAA.
[ 22 August 2001: Message edited by: G.Khan ]