Doesn’t matter how “happy” the dealer is you can cancel at any time up until you input your pin and according to the Volvo sales reps they are selling like hot cakes so most are refusing any kind of discount and if that is the case then they will have no problem selling the car on straight away.
1st point of contact is your supplying dealer (just tell them your circumstances have changed) and then I would contact Motability and inform them as well as dealers have been known to ignore cancellations before and try to bully people into signing for a car they no longer want.
Most importantly though is the fact it is your money and your choice and if you think this car will not but suitable to your needs for the next 3 years then do not worry about it and just cancel.