The Border and Immigration Agency has not made any estimates of the number of A2 nationals likely to enter the UK for employment purposes following full right of access under EU law. Figures on migration since January 2007 are available in the quarterly releases of Romanian and Bulgarian accession statistics which can be found on the BIA website
http://www.ind.homeoffice.gov.uk/aboutus/reports/bulgarianandromanianaccession.
I will place in the House Library copies of the most recent quarterly release.