Hi, @Sydneygirl!
Thanks so much for posting your packing list. I live in the U.K. and I travel around it with my 3&6yo a lot (mostly by myself) so I am really into your question.
The thing that leaps out at me from your packing list is an abundance of shorts. Last year we did have about 6-8 weeks of sustained temperatures above 20 degrees C, but it isn't normal and especially if you are going to Scotland you're going to want to ditch one or two pairs of shorts, add long trousers, and also wellies.
I know, wellies take up an enormous amount of space in a suitcase. Could you have your kids wear them on travel days? Or order some cheap ones to be delivered to where you're staying? Once you get here, you are driving, right? So you could buy them here and leave them in the car. There are really lightweight ones here of Demar brand (ETA: see link below). They're the lightest weight ones I've found. Or you could go to a shop, a Sainsburys/Asda/Tesco big enough to have a clothing department might have some.
Wow this post is getting long. To sum up, a merino layer or even a packable down jacket, and more long trousers/long sleeved tops, and closed shoes or wellies for the kids will serve you well. It has been 8-12 degrees here the past few days, and this week coming it will be 12-16 where I live. As I said, last year at this time it was in the 20s every day. Not so this year.
I hope it warms up a bit for your trip!!
I almost want to make a separate reply for this but what's your in-flight bag? Are you also planning on keeping the entertainment and snacks and stuff in the Aeronauts or do you have separate personal item bags for that?
ETA: here is a link to Demar weeklies. Make sure it says ultralight or similar in the name.
https://www.amazon.co.uk/Ultralight-.../dp/B013REDOWG