As for the survival in the British Isles idea. Much as I like the thought of my home surviving, I just don't see it happening, definitely not on the scale you are suggesting. As I've said before, this country is far too international to make any attempt at quarantine effective, plus our politicians are too timid to enact martial law to maintain order. If there were survivors it would just be tiny groups that were forgotten about/passed over rather than a population the size you are suggesting. Still the thought of us reverting to a monarchy makes me smile.
Well in the comics I recall that Sweden and Finland implemented their closed borders rather late in the day, around day 9 I believe, I get the feeling that they did so because other larger EU members were beginning to do the same. France borders Spain where the infection would be strongest, hence they'd close their borders, Italy and Portugal would be likely follow suite, then once Germany does it the whole of Europe will go into lockdown. Britain may stay open for an extra day or two just for the sake of image but the government is bound to realise that at some point that maintaining trade whilst the infection is in full swing is simply damaging to themselves and to the rest of the world.
As for martial law, as an Irishman I think your underestimate your governments willingness to shoot its own civilians
There will be terribly huge infection rates and deaths, but like the Redeker Plan in WWZ I believe a small "vital" population could be protected in the more inaccessible areas of Scotland, whilst other Britons are evacuated to infection free Ireland. Though even this won't be perfect since the evacuated population will be too large for the designated survival to support, hence there will be famine, sickness and lots of fighting. I like dystopia though so I think that's cool (:
I had actually been thinking of the Norwegian, Danish, and Swedish royal families at the time. Minna hasn't mentioned them and the only government we've heard of has been the "Nordic council", but I'm sure they're alive somewhere. And this of course got me thinking, and I eventually came up with the image of King Haakon VIII, the Troll Hunter. And if the Scandinavians are protecting their monarchs then you Brits will probably keep your beloved pensioner safe during the crisis. Her issue is just taking a more hands-on "old school" approach to government, which also involves flogging thieves and "evisceration" for traitors (that's getting your stomach cut out.) Monarchs/dictators work best in a crisis, that is if they're smart anyway, and the British monarchs have been raised as military leaders, hence they're pretty smart.
Kapitoid, that's some pretty interesting speculation about the state of the British Isles you've got there (nice map too). While I'm not sure whether I share Koeshi's concerns about martial law being viable in the UK or not, I do have one question: how have the British and the Nordics not made contact yet? I'm looking at google maps now and the areas you've marked as safe/cleansed aren't too far away from one of the shipping lanes on the Page 66 map. I would have thought at least one British trawler or something might have run across a Nordic ship in 90 years or so.
That's the one major flaw with this isn't it?
I'm gonna say sea beasts, which is a cop out but I get the feeling that the Nordics are focused on keeping the trade lanes open and not on exploring the islands to the south which to their knowledge are likely crawling with Trolls and Giants just waiting to eat them alive. I don't believe the British would even have the military resources in Year 90 to go exploring, I imagine their life as very day-to-day just trying to survive and stop the Union collapsing into a civil war. You remember the Nordic Council lady who said the expedition was a waste of time? The British government is definitely in agreement, and they have a reason to say that. If they go off their respective beaten tracks Sea Beasts which live in "uncleansed" waters will nom on your little boat.
Though the British government is likely aware that there would be other survivor communities around the world. Just they imagine they're probably doing worse than themselves and contact would just lead to hundreds of people begging to be evacuated.
Also, why on earth did that navy ship you mentioned go rogue?
As Koeshi suggested, a captain eventually decided that enough was enough. The British navy is huge compared to the Icelandic one, and unlike the Icelandic one they are killing the civilians they are supposed to be protecting to keep a completely different country free from infection (never mind that 400,000 English people are already being kept safe there). Dissent is bound to take hold on at least one ship, in this one it just happened to be off the coast from Dublin where it let two fishing boats through and nearly sunk an Irish ship to let them make land fall. In fact I wouldn't be surprised if more Royal Navy ships just decided to go pirate and sail off for the Caribbean. War is hell yo.