If you’re on a road bike and want to stay on tarmac, flick the switch from ‘Paths & roads’ to ‘Paved only’. This way, the route will prioritize smooth surfaces and avoid hilly sections.

Thanks Shane, I appreciate your response.

I'm not so sure that choosing "Paved" as opposed to "Any" surface will dramatically change climbing. Surfaces can make the experience of climbing different though, in my experience.

It's built in to CT to minimise climbing but if we choose to go from A to B and there's a big hill in the way... 

Perhaps the key is to look at how much we are interfering in CT's routing by being careful where we place any viapoints.

On the website, at least, we can normally ask CT to offer an alternative.

We can ask CT to create a round-trip, effectively giving us a second way to go (just reverse the direction)

If choosing 'suggest a ride' we can normally choose from 3 options (and sometimes more if we hit the button again). It's relatively easy to compare climbing across the options.

The hill sensitivity is a bit different for the gravel mode but otherwise the same on all other modes.

cycle.travel does try to minimise climbing, and steep gradients in particular, but it weighs it up against the other considerations (car traffic, surface, scenery etc.) which means that it won’t always choose the flattest route. Unfortunately the way that the route-planning engine works means that it isn’t feasible to add lots more options without a heavy investment in renting more servers.