That last sentence surprised me! Could you clarify please? I thought it was a "rule" that all flights had alternates for the whole trip. Is it different aircraft type or different airlines that dictate the use, or not, of alternates? Or is it a London thing?
An aircraft never becomes constrained for fuel until nearing the end of the journey. For the vast majority of the flight, I'll have any number of places available. In fact I'll have so much fuel, that I'll be overweight for much of the time.
But, as you near the end of the trip, most of the fuel has been burnt, and you are falling back towards your minimum fuel. Min fuel was discussed a couple of items ago.
I need an alternate if the weather at the destination is below whatever 'alternate criteria' have been published for the airport. That will vary depending upon the equipment available on the aircraft and at the airport. If the weather is below for a nominated period on the forecast (INTER or TEMPO) then it means it will be below the criteria for 30 or 60 minute periods. I need to have either alternate fuel or holding for the nominated period. Sometimes, with a close alternate, the alternate fuel required might be less than the holding figure.
Basically though:
a) weather outright below the alternate criteria, must have an alternate
b) weather INTER below, 30 minutes holding
c) weather TEMPO below, 60 minutes holding
d) weather above the criteria. No alternate required.
Some airports have standing holding requirements in addition (most in Oz need somewhere between 10 and 20 depending upon the time of day). But that's for ATC to muck you around....
So, the rule...is a myth.