It did take me a while to get around to it because admittedly I'm very shy and awkward in these situations. My boyfriend and I went up to the new Boots at the Kingsway retail park, which was much quieter and the lady from the No7 counter actually approached me. That made it much easier.
So basically she wiped off my existing foundation, and used the little device to take a photo of my skin on both sides of my face. It turns out it came up with a warning saying my skin couldn't be matched (my boyfriend found this hilarious). I wasn't all that surprised really, I realise how pale I am. She did manage to get a reading from the left side of my face, it was for the lightest shade they have, Calico. This was then applied to my cheeks on either side and she blended it in, but it was still far too orange for my skin tone. After a couple more tries she suggested their Intelligent Colour Foundation instead. This worked much better, it's more sheer and comes in the shade Extra Light. Which she informed me was a new addition to the range.