With my recent trip to the UK, when I handed my passport over at the check before boarding, the customs officer rifled through my passport, and when he couldn’t find a stamp, asked me where I lived. I responded France and handed over my CdS and then he was all smiles, gave me a big Merci and didn’t stamp my passport. When I returned to France, I joined the far far shorter EU line and when I got to the checkpoint, duly handed my passport over, with CdS and the customs lady again gave me a big grin and waved me through. Seems like they feel anyone with a CdS has been ‘converted’ and on side 
All in all, a pleasant, and very simple experience 
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