I don't think any country has thought this through, mikey.
So far, all I've thought about is whether our sewers can cope with the sudden insertion of a medium sized town into Britain.
Lucky we've had a wet summer though - an extra 20,000 showers/baths/ritual washings per day, plus other domestic consumption. There used to be a statistic bandied about that we have less drinking-quality water per capita than the people in the Sahel do. It may be time to fact-check that now.
The laws of supply and demand mean prices can only go one way.
Disclaimer: 20k extra Brits would have been added to the existing population in any event but at least that would have entailed a suitably long time of adjustment and infrastructure expansion. It doesn't matter who they are, it's the suddenness of it all that I don't think we're equipped for.
Germany's change of attitude was sudden and perplexing, given their earlier stance.