A belated thanks for the replies to this. The place in Strutton Ground is indeed an excellent place for Loakes, and I do call in occasionally, but they don't stock the Pimlico (and don't seem willing or able to order in). I ended up ordering online (though even finding the Pimlicos available on the internet was a challenge).
Glad to see that Loake have finally opened a dedicated store. Seemed crazy that it was impossible to buy a model from a major English maker anywhere in London.
On the subject of the Pimlicos, they are just what I was looking for style-wise, but a couple of months into wearing them once a week, they're still bloody painful, mostly around the ankes (and I've worn many other chukkas).