If you think of training as replicating, then the best training is the most accurate replication. Hill reps will assist in pushing your fitness but its not as good as long sustained periods if that's what you are tackling.
In the Surrey Hills, despite the cheesiness of it, Box is better than some of the other bigger steeper hills for making you push through at a sustained rate like you will face, than a collection of shorter shaper kick-ups. Unfortunately you're looking at 7-8 minutes rather than hours and, like stated, coming back down gives you time to recover. Looking for good routes that will keep your HR up are good but the resistance on the pedals isn't the same - you could counteract this by pushing into one higher gear than you're comfortable to slow your cadence a little and increase the resistance perhaps?
In the Surrey Hills, despite the cheesiness of it, Box is better than some of the other bigger steeper hills for making you push through at a sustained rate like you will face, than a collection of shorter shaper kick-ups. Unfortunately you're looking at 7-8 minutes rather than hours and, like stated, coming back down gives you time to recover. Looking for good routes that will keep your HR up are good but the resistance on the pedals isn't the same - you could counteract this by pushing into one higher gear than you're comfortable to slow your cadence a little and increase the resistance perhaps?