Ferry pricing can be quite complicated.
On my last two trips back to the UK it turned out cheaper to pay more. The first time because the "overnight" ferry docked at 3 am, and I dont call that a nights sleep.
I ended up using Brittany ferries instead of my usual LD lines, they are longer crossings, but I arrived at my destination early in the morning having had a nights sleep and breakfast. Travelling overnight can often save a day. To get the timings right for me I had in both cases to depart by one route, and return by another. This counts as a return with Brittany ferries, but as two singles with LD lines. Making the poorer service more expensive. Also when I factored in the travel costs (mileage), LD lines were again more expensive. Until recently they were usually the most cost effective, but twice not so this year. ( actually three, as we will be visiting the UK again soon).