The Portsmouth to Le Havre ferry crossing is excellent for getting a sleep before the drive on Saturday morning especially if travelling from the north of the UK, you also save the drive from Calais to Rouen.
However it is expensive. There are cheap deals but they go fast and take some finding. We chanced on one of the £39 each way deals this February so they do exist. We think they have a small quota, maybe only a couple per journey? Best we can recommend is do a search daily some time in advance and see if one appears.
Flexible ticket or not?
It is good to have spare time when coming back (and going) but you have to pay a premium. If you miss your booked ferry you can get one up to four hours later.
Alternatively why not book a later crossing anyway and turn up early? Most of the carriers seem to just let you get on the next available ferry – we have never been charged extra for this and the extra they may charge isn’t a lot different to the premium you would have paid – or if it’s too much just park up and wait or do some wine shopping.