App won't use Saint-Malo to Portsmouth ferry line, makes up a fictional crossing

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5.0.9 Android
My plan goes from Southern France to north London in the UK. I've added the departure and arrival harbours, as well as adding the proper ferry crossing (Saint-Malo to Portsmouth). The result is totally wrong and sends me to various channel islands, sometime several times, ending up after that driving hundreds of miles to the Channel Tunnel. I realise I could just do two different plans, one for the French side and one for the UK, but…