The flex option is great if you want to start on a basic plan or on a plan with a high deductible and consider upgrading later to a better plan or a lower deductible.
Flex allows you to make an upgrade after 3 and 5 years by:
Reducing the deductible. You might want this if you develop a chronic condition and have high yearly costs.
Upgrading to premium. You might want this if you want to be able to go straight to the specialist rather than having to see a GP first.
The flex option doesn't make sense if you're happy with your plan and you have a low deductible.
It's great if you just want to sign up for "the cheapest option" for now, and then reconsider what cover you want long term later.
If you don't have the flex option, you can also upgrade, but you'll have to go through a health checkup (at your own cost), and it's possible the upgrade will be denied depending on the outcome of the checkup.