What would stop trolls going over to scottland? It is not known for really cold winters in comparison to scandinavia or the alps for example.
You'd be shocked how cold northern scotland can get in winter. The south of Scotland will probably be trolled up to the eyeballs especially around edinburgh. But it doesn't seem to me that trolls are incredibly inclined to roam. They seem to have nesting habits unless they come across something that they want to kill in which case they follow it.
So for the most part while some parts of northern scotland will have substantial troll populations courtesy of southern refugees the majority of northern scotland will be left almost untouched due to the sparse population and lack of transport networks (including roads) making it difficult for refugees from the more developed south to reach these mostly isolated communities.
Your image of a giant beast blop covering london is amusing as well as scary. Imagine how İstanbul would fare 
In my opinion one of the first things our government will do is go "oh dear those poor refugees, let them all in!" especially if its a Labour government at the time. Meaning instant widespread infection. And even if by some miracle they don't, our government will not be able to control the borders when faced with the huge numbers of refugees fleeing the plague, with London being the primary destination for them.
What would the welsh eat though? Same goes for Scottland. Any infested person would turn sheep and the like into trolls. Chickens are not a major ressource and getting hand on fish means climbing down the mountains.
There's plenty of edible plants that grow in Wales. The image of it being nothing but sheep farms while somewhat accurate is not entirely accurate.
Their primary defence though will be the unforgiving terrain in many parts of Wales. Much like it has against invading human armies it should provide a considerable deterrent to trolls who might be inclined to wander rather than nest. When they do get in, in numbers though it will probably result in the loss of an entire valley. Like when the English conquered Wales, it was done one valley at a time.
Another problem is that Wales and Scottland are well known tourist areas. If I head to flee to some other country knowing I might die there, hey I really would opt for northern Ireland (death at the cliffs of moher or the giant's causeway, why not?), the orkneys or Wales. Also the english fleeing would try to go to these areas and I am not sure how much military is available to hold them back.
In both Wales and Scotland it is more the south that is strongly associated with their touriest trades. Northern Scotland is more agricultural and northern Wales a mix of agriculture and old destitute mining towns. Not exactly places people will flock to.
What's important to remember though is population hubs, transport links and most importantly the inclinations of each regions political class and their locations.
Northern England will have few places to flee as southern Scotland will go bad fast due to their governments knack for making terrible decisions. To their south they'll have the London blob and refugees fleeing that. While they could flee west into Wales many will find that there's a few population centres between them and Wales.
So they'll be pretty dam trapped. In all probability you'll wind up with a northern line in Northern England that is relatively safe centred around Yorkshire and any old town or city with their old walls relatively intact. Then with a safe zone in southwest England where they've been shielded from refugees who fled north and refugees who fled the continent by the various channel islands.
As for Northern Ireland. Well I wager once things start going bad the IRA will be back in action almost immediately which of course means the Loyalists will be picking up arms again. Making Northern Ireland either a very safe place in the long run or a very very very bad place to ever be.
Poll
