With all emergencies you do what is right at the time, nobody is going to tell you off for using what ever comms are to hand if someone's life is in danger.
Regardless of wether you use VHF or 999, if you're on the broads you should get handed to Humber...
Generally the lifeboats covering the broads know the best places to launch, as long as you give an accurate location they will find you.
It's always good to have a plan. If you or your crew are senior & you moor up some where that has no phone signal, you don't have a VHF and there are no other boats about you have created a perfect storm. My mum always said 'failing to plan is planning to fail'