The former have not been picked up by the public authorities though clearly exist as EU citizens can move into the UK without registering with the authorities. While the latter were certainly around at the time of the last Census in 2001 but were not picked up at all. This is not surprising as many of them will have been fleeing police states in the Middle East, where it would be considered very suspicious to give such details to a public authority even though it was something like the Office for National Statistics (ONS). This was a particular problem in boroughs like the City of Westminster in London, which in large part explained the under reporting of the numbers through the Census in 2001 going by other indicators.
Lets just hope such under reporting does not occur with these emerging diaspora communities as well as any others coming through in this years Census. Otherwise London will go without critical resources for all its residents and not just the new ones.