What was the UK like in the 1000's?
A few million.
Roman Britain is estimated to have had roughly 3 million or so. That likely declined with the fall of Rome, such that it might have been a bit below that by 1000. After 1000, though, the population expands steadily for three centuries, reaching a medieval height of c. 4 to 6 or 7 million. Then came the Black Death, from which the population didn't recover till a few centuries later.
Note though that the population of what would become the UK was relatively small during this period (before the Industrial Revolution). France for example often had three or four times the population the UK did.
Cities in Europe were also much smaller, even comparatively, due to logistics and low crop yields. In 1000 AD, there were no cities larger than 100,000 in Western Europe (excluding Byzantium and Muslim Spain). Even the larger ones north of the Alps, such as London and Paris, were very small by modern standards -- on the order of 20,000 or 30,000. The largest in Italy (Milan, Venice, etc.) were probably approaching 100,000 by the end of the high Middle Ages (13th century).
By those standards, a city of 20,000 or 30,000 could very well be the capital of an entire kingdom, and a city of 50,000 would be the medieval version of a metropolis.