We did LEJOG bikepacking this year, staying in B&Bs. We called home half wayish so it was 2 x 5 day tours of about 90 miles per day. So I have some experience of exactly what you are wanting to do
I bought one of these:
BACKLOADER | Topeak
10 litre is available about £45. It was exactly what I needed. I also used a Topeak toptube bag. Bags plus all kit was about 3.5kg.
Kit list:
Spare cycling top
Spare cycling shorts
Spare socks
2 t shirts
1 pr lightweight trousers
1 pr nylon shorts
1 pr flip flops
2 pr underwear
1 lightweight showerproof jacket
2 inner tubes
2 CO2
CO2 adaptor
1 pump
Spare tyre (bit excessive but we were in remote areas of Scotland)
Phone
Garmin
Sunglasses
Charging cables, USB plug
Toothbrush, travel toothpaste. No soap etc B&B will have
I adjusted my bottle cage mounts so I could fit 2 x 1 litre bottles under the top tube bag
multitool
I think you've got enough clothing to handle Scotland in poor weather. Cycling shirt if it's warm. T shirt, Cycling shirt, waterproof if it's cold. Could take some longs and maybe some lightweight overshoes. We did LEJOG in July so even when it was wet it wasn't cold so always rode in shorts
The thing I didn't take, but I'm very glad that
@I like Skol did, was a rag and some chain lube. After a wet day the chain needs fettling to keep it running smoothly