It is the British housing system.
William A.V. Clark, Maarten van Ham, Rory Coulter:
Socio-Spatial Mobility in British Society
The analysis shows that education and income play critical roles in the ability of individuals to make neighbourhood and decile gains when they move. There are also powerful roles of being unemployed and being (and becoming) a social renter. Both these latter effects combine to seriously restrict the possibilities for socio-spatial movement for certain groups. The results suggest serious structural barriers to socio-spatial mobility in British society, barriers which are directly related to the organisation of the housing market.
http://ftp.iza.org/dp5861.pdf ===================
It takes a little effort to get past the academic language, but only a little. Deciles are tenths, so moving from the 9th decile to the 8th is a gain and moving from the 9th to the 10th is as low as you can go.