Not that I am a world expert on the subject but I've made a few trips of several days and a few thousand miles each.
You've already got the best advice about checking the bike: check everything you'd check during either a normal maintenance session. I find it less worrisome to take care of scheduled stuff before departure, if I can and if they are within about 10 percent of the margin for needing it, so that I don't have to think about it while traveling. That last bit of wear on the tires, ("tyres" to you...) , or a little while longer on oil, coolant, and brake fluid isn't worth the mental clutter of knowing I have a change to make as soon as I get back home.
GEAR
Rain gear. Like tools, take it to avoid needing it.
(So far I am 0-for-4 in that regard- I've encountered at least a splash of wet weather on every long ride I've taken.)
Depending what you ride in, alternative footwear for your pedestrian excursions, and for evenings.
Fewer changes of clothes than you'd expect. Lightweight, fast-drying fabrics preferred. Easier to wash in a sink, and much less bulk to manage when packed. Except for socks and underwear, if I air my things out for a day after wearing, that's good enough for another day on the road later on, before washing.
STRATEGY
An open mind and a flexible schedule most of all.
YES.
Stop when you want to, don't be a hero.
YES, again.
Read some of the ride reports (especially from HPPants) and notice how often a day starts from wherever the previous day ended, and has a general but not specific destination in mind. That leaves time and opportunity for "discovery" and unplanned diversions, and eliminates the stress of feeling as if you "have to" be at a certain place at a certain time. Be on the lookout for roadside attractions and diversions. Roadside historical markers (do you have those in Britain?) can be very entertaining and provide opportunities for short, unplanned rest breaks.
Perhaps not applicable to this trip, since it looks like you will be in transit then doing loop rides from a fixed location while in Scotland, but on other wanderings: pick your final destination partway through the afternoon of each day.
Spur-of-the-moment jaunts down inviting side roads can be very entertaining and enjoyable.