I'd recommend a sailboat.
There's zero fuel requirement and since you're in the UK, you've got plenty options of other places to go, like France, Spain, Portugal, or
anywhere through the straights, or even down Africa.
Whatever is happening to make you want to shove off, there'll be options for landfall.
You could, if you wanted, even point West for the Caribbean, Rio De Janeiro, or someplace where you'll never see Winter again.
With a sail and proper kit like a wind generator, some solar cells, a watermaker to convert ocean water into potable fresh, as well as enough
supplies, you can go anywhere.
Just making it into the Mediterranean gives you thousands of options from attaching yourself to the tourism industry in some holiday spot, finding a
nice little island like Majorca to call home, or just staying on the move in stopping one place for a week or a month, or however long and moving on
to someplace else to move on again.
Whatever the case, you have flexibility and you can carry much more supplies in a sailboat than you can in a backpack, plus, if needed, the ocean can
and will provide food if you know how.
There's also hundreds of little islands all over the Aegean Sea, some of them even vacant except for like wild goats and rabbits because they suck
for human habitability but suit the goats just fine.
Find a place like that and you've got some Gyro meat.
As I've said elsewhere, a bunker is one thing, but, by definition a bunker separates you from a threat with several feet of concrete, steel, earth,
Several feet of protection is fine and all, but, I'd prefer several hundred miles of open air between myself and any threat.
Several hundred miles of open air, in my opinion, gives far more protection than even a mile deep bunker.
Call it an open-air bunker if you will. Go Sailboat!