Invest in a full service and obviously anything that looks worn or likely to fail should be replaced beforehand. At the very least make sure you carry a basic toolkit, especially if the bike is chain driven as this may need to be adjusted en-route.
While modern bikes are very reliable some basic spares are always handy, like bike levers just in case the bike gets dropped, but don’t get carried away and load yourself up unnecessarily, Just invest in a good service beforehand. Carrying a set of spare bulbs is compulsory in most EU countries. A spare throttle cable is a good idea too.
Duct tape and cable ties are useful for dealing with emergencies, broken fairings , dangling indicators etc and do not take up much space. A small canister of chain lube and water repellent spray are also useful additions, as is small sachet of hand cleaner. Self-sealing (amalgamating) silicon hose repair tape is a good idea if you have a water cooled machine and this can avoid you being stranded by a leaking hose.
Make sure there is sufficient wear left in your tyres to do the entire journey and get you home again without the tyre becoming illegal. Running a set of tyres to the very limit is a false economy that could end in disaster on the final (usually wet) motorway stretch back home in the UK. Adjust the tyre pressures to deal with the extra weight of luggage etc.
You should carry a spare tube or a tyre repair kit if it is a tubeless tyre, to allow any small punctures to be plugged to enable you to get to a bike shop for a permanent repair or replacement, rather than be stranded for ages waiting for help. The aerosol type repair-and-re-inflation cans are very good value and easy to carry/use, we always keep one under the seat as well.
Check your brake pads have plenty of life left in them, and if not replace. Check all of the fluids and hoses etc, and that all electrics and switches work as they should.
The suspension should be adjusted in-line with the manufacturers recommendations to take account the extra weight of the luggage and or pillion. A spare key is always worth taking, especially if you have a pillion who can keep it, or you can let your tour guide keep it safe for you.
If you are using a ‘naked’ machine consider fitting a screen (eg Puig or similar) to reduce wind buffeting on long journeys. Also consider heated grips if travelling at colder times of the year.
Unless your number plate incorporates a GB identifier you will need to attach a GB sticker to the rear and you will need a headlight beam deflector at the front to dip to the right avoid blinding drivers at night.
A decent bike heavy duty lock and chain or at least a good disk lock is also a must.
Go for a trial 2-3 hour ride. If your bum gets numb then get a gel seat or an air hawk. Riding six hours a day for a dozen days with a sore arse is not what it’s all about !