AIP GEN 73.2.1
Except when operating an aircraft under the VFR by day within 50NM of the point of departure, the pilot in command must provide for a suitable alternate aerodrome when arrival at the destination will be during the currency of, or up to 30 minutes prior to the forecast commencement of, the following weather conditions: blah, blah, blah
Nothing in the entire AIP section regarding Alternates states that these are only pre-flight requirements. This means that at any time during the flight your ETA falls within 30 minutes of conditions below the alternate minima, an alternate must be provided for.
Understandably, if before you departed there was no weather forecast below the alternate minima you probably won't have the fuel to carry an alternate from your destination if the weather does deteriorate. If this does happen you will probably have only two options:
A) Divert to an enroute alternate, before reaching your destination to uplift additional fuel
or
B) if there are no suitable airports within range, declare a Pan-Pan and continue to your destination.
Of course, if you can hold, hold.
With the mob I work for, on sectors in excess of 1000nm we nominate a point between two airports. If after we have passed this point the weather at our destination falls below the alternate criteria and we have no other options available it's accepted that we may continue. Before this point, however, if there are no other options available to us (eg holding) we must divert.
My understanding of the AIP is that this is pretty more or less the concept that everyone should be adhereing to.
For those of you in the know or curious about that policy, I acknowledge that yes, our policy is a little more involved and flexible than I've just stated but I've kept it simple, for argument's sake.