This article in The Telegraph last year explains that when waiting lists are long, councils should provide more by buying land for it.
Councils exploit legal loophole to get out of providing allotments - Telegraph
extract -
"Under the 1908 Allotments and Small Holdings Act councils have a duty to provide land if six people or more request an allotment. Should none be available the authorities are required to force private landowners to free up space.
But Donna McDaid of NSALG said there is no time limit on how long councils can claim they are actively "looking for land" and some say they have searching for up to eight years.
She said this preventing campaigners from using the law to force councils to provide allotments because officials claim they are still looking for an appropriate site.
If people are asking for allotments then councils have to look for land, but a lot are just saying they are looking and we can do nothing about it because there is no time frame," she said. "I think they are probably exploiting a legal loophole."
In the face of this dilemma it seems that the council wish to reduce, or even abolish, the waiting list by making allotments unaffordable for most people.