I don't suppose anyone would know how closely Brittany Ferries scrutinise car heights on the Portsmouth - Cherbourg route? Just booking the Summer holiday ferry and the roof box takes us 13cm over the lower/cheaper height bracket (<1.83cm) and costs an extra £55.

Thanks

Never seen them checking...
But, if you have booked the lower height, and they see a roof box, they may get more interested

Apologies for not acknowledging sooner - how rude! Was considering letting some air out of the tyres but in the end have opted to pay the extra for a stress free life!