I've just done a quick bit of research on some of the countries already mentioned in this thread:
Sweden has 36% of its population concentrated in just 3 urban areas. The UK has less than 25% situated in its top 5 urban areas. So, as I suspected, Sweden has a much higher concentration of its population in a smaller number of urban areas - meaning there is less external infrastructure to reach that proportion of the population. In fact, you have to include the population of the UKs
20 largest urban areas to get up to 36% of the population, giving a much greater cost to provide infrastructure to all those areas.
The Netherlands has a base population density of 405 people per square km. The UK's figure is just 256. So people live much closer together in the Netherlands, which again will make it much easier to provide high speed broadband to a higher percentage of the population.
France and Germany, as far as I can see, both have generally better speeds than the UK - apparently France's copper cables are better quality than ours (!?). Germany is interesting, because it has a very similar population density to the UK but a tiny percentage of residents in large urban areas, suggesting the population is fairly evenly spread over the entire country! Clearly more research/thought required, but it's nice to see that some of my gut feelings on the difference between the UK and Netherlands/Sweden turned out right