Oh dear - this thread needs moving to spotters or JB really.
Anyway
Unless you know of a way (not involving a pan or mayday) to force ATC to let you fly the company planned diversion route, while everybody is diverting as well, you will quickly encounter a fuel pan pan or mayday in order to ensure that you do fly the planned route and land with 30min.
- looks to me as if you have answered your own question. Yes - you TELL them you are setting off for the alternate. It is called 'command'. Safety comes first, ATC later. Their job is to provide the service. Assuming you have asked them first and they have refused, then you declare a Mayday.because you
will be below F Reserve. It is not difficult.
Go on, be brave - start a thread on Safety CRM and QA about EUOPS fuel planning. I'm sure you will get interest.