Unless it is high summer (and sometimes even then) you will want to get picked up fairly quickly (hours at most) to have a good chance of survival in the channel.
If you do not have a PLB this will be down to luck. A dingy (or worse a floating man in a life-jacket) is not easy to spot. Even if someone has a good radar position for you.
Of course if your ditching goes off very well and you manage to get into a dingy without getting wet and/or have proper survival clothing then you have more time to play with.
How many of us wear immersion suits to cross the channel?
A PLB is cheap insurance.