^^ heh!
do you mean local authority housing by any chance? I seem to remember that if two families wanted to move (who were in council-provided houses/flats) they could approach the council to arrange a swop? Remember seeing it happen alright between say a family of 4 in a two bed apt in city centre and family of 3 in larger house but on outskirts of town who wanted to be closer, so arrangements were made for a direct swop, but this was only if the two parties were local authority tenants.
NOt sure if this is still in operation though?