Whilst the figures say one thing i.e. 30,000 issued with an NI number, there is nothing there to say if 30, 300, 3000 or 30,000 have any form of gainful employment or indeed any form of decent housing.
If they have left the family home in Spain they may well be living in far worse conditions here, maybe one room in a hostel in the UK. There is virtually no social housing left at all so unless you can find work and therefore rent a place in the private sector the chances of getting accommodation are slim to nil unless they bring 11 kids with (presumably something your average Spainish young person will not have achieved nor desire).
I know of someone who was put into one of these 'holding hostels' that under no circumstances folk are supposed to be put in for more than 4 weeks and is still there months later

and those in the rooms next to are not what you would call dream neighbours

.
It's hardly living the dream either but it is often all that a national insurance number will get one.
In terms of unskilled work for people with English as a second language, the Polish girls have 80 to 90% of the waitressing business sewn up and excellent waitresses they are too, fast, polite and accurate with billing, potentially in a different league to a Spainsh waiter or waitress who are just not as fast as UK restaurant clientel require, the other 10 to 20 % tend to be Brit Uni kids working to support their education or those who are trying to work their way into senior levels and Management in hospitality and catering who have undertaken a two year Btec in H&C but are starting at the bottom and working their way through.
It's not just about raw data, there is a qualitaitive issue as well.
You will never stop people spreading their wings and trying another Country it is something folk have to work out for themselves and has gone on for centurys, provided a Brit has a plan b there isn't much that can be done to stop them, nor should there be, a few will make a roaring success of it and so will a few coming the other way as well.