Most prisoners who are lifers or serving very long sentences tend to get single cells from the beginning (at least when they are in the high security (A-cat) dispersal prisons. Other single cells are reserved for what are considered to be high risk prisoners (particularly those with a previous history of violence against a previous cell-mate). Otherwise, it tends to be buggins turn as and when singles become available. In fact, owing to the current shortage of prison places, many single cells have already been converted to tiny doubles.
I know quite a number of prisoners who have refused to share cells with others they know or suspect are violent or who have a current drug problem. It is always very risky to share with a drug user because their stash can get hidden in your property or under your locker etc, potentially leading you to get charged if it is discovered during a cell search ("a spin" as it known).
|Standard Prison Bunk-Beds|
Only the most recalcitrant of prisoners refuse point blank to share a cell and they risk being charged with refusing to obey a lawful order and being up before the governor on an adjudication - which can lead to loss of wages, gym access, canteen purchases, association etc. It is also possible to end up on Basic regime (solitary confinement, little personal property etc). I've known this to happen, but it's not common.