They are Audax Rides, run by Wessex CTC:
Dorset Coast Dorset Coastlet
Wessex CTC website, has some more info.
I was entered for the Coastlet, but have withdraw due to a problematic knee injury, have been advised to steer clear of anything hilly
Either ride should take in some stunning scenery, it's a beautiful part of the country - I'm painfully familiar with the Abbotsbury/Hardy Monument stretch and it's associated hills, but the views should almost make up for the climbs!