One of the things we noticed for Paris Brest Paris 2023 was that people who had completed a Super Randonneur Series the year before were:
a) more likely to get through the qualification process
b) more likely to complete the event successfully
Whilst there is no official qualification for London Edinburgh London doing a Super Randonneur Series this year will better prepare you for your event.
If you can spend one year building a solid fitness base this will mean you can build on that fitness next year and your training will be with less injury risk.
If you spend a year finding out what works and doesn't work for you in terms of training are far more likely to nail it when you need to in the year of your event.
Practising your practise events increases the odds of you completing them when it counts
The gradual progression from 200km to 300km to 400km to 600km is a logical progressive step. Doing them under the Randonneur Mondiaux code gives you an idea of timings and demands and doing some or all of them as official events will get you used to the way the events are run.
But long rides are not just about fitness. Getting fuelling, equipment, hydration, sleep and dealig with mechanicals are all part of what makes you able to complete an ultra bike event.
Practising your practise events increases the odds of you completing them when it counts.