LHR-PER is likely to be payload limited every sector holding YPLM or YPAD as an alternate, and probably not economically viable. Another consideration is the frequency of a TEMPO, once again reducing yield.
The real killer is departing on the basis of clear forecast, given the sheer sector length, statistically you are simply more exposed to a adverse change in the forecast.
Given the flight plan is likely overhead Columbo and then heading south to pick up the jet, there won't be many alternates available, and the decision point is going to be quite a few hours before PER leaving a large window of exposure to a change in the PER forecast without a close alternate. The DP may even be before the ability to get a TTF to cover the arrival time.
Here is good
Great Circle map of the LHR-PER route with 1400nm range rings from Jakarta (WIII) and Learmonth. As I said, it is likely the actual flight plan will probably be well south of the great circle track after Columbo, and then almost due east.
A guess the bottom line given the geometry is that this probably about minimising the number of early diversions when the TTF ends up being clear, thus maximising payload. Just a working hypothesis based on a bit of guesswork.
