Hello, I am also based in the UK. As soon as I saw rumours of the caramel mini, I messaged my SA straight away. This was a couple of months ago. I would then normally send a message to my SA nearer the launch date to see if they'd be getting the bag in. She has always been really helpful and has managed to secure me the bags that I want.
I have never tried going to the boutiques on launch day as the queues are sooo long and there is no guarantee that you will get what you want. Try and reach out to SAs that you have previously clicked with and see if they can let you know if they are getting the bag in. Most bags are reserved before they hit the shop floor, but obviously no where near as crazy as USA. Hope this helps!