Many of those who are given day release (other than for compassionate purposes) are already in 'open' prisons. For example, prisoners at HMP Hollesley Bay (in Suffolk) could simply walk across the neighbouring farmland if they wanted to escape. It rarely happens because most people would rather complete their sentence (which is probably near to its end anyway) than spend the rest of their life 'on the run'.
Quite a few prisoners from HMP Hollesley Bay have jobs in local firms (on a basis similar to college 'work experience' but over a longer period) to help prepare them for release. Because the prison service find it hard to get such placements, some of the jobs aren't really that 'local' and it's not unusual for prisoners to travel, unaccompanied, by train, into neighbouring counties, in order to work. They have to carry a copy of their day release licence with them at all times and they're subject to strict rules about where they can go. (They may be allowed to carry enough money to, say, buy a coffee from the work's vending machine but they can't go shopping or off to the pub). They also have to be back at the prison (or at a point where the can be met by the prison minibus) by a fixed time. If they're genuinely delayed, they're obliged to obtain proof of the delay (e.g. by asking a railway station supervisor to write a note stating that the train was delayed).
There aren't enough places in 'open' prisons to accommodate all of the prisoners who meet the criteria for 'Category D' status (which is the lowest security risk). So those prisoners who meet the criteria, but are still housed in more secure prisons, may also be offered the chance to work outside prison. However, this happens less often simply because the more secure prisons often don't have the staff who specialize in finding work placements (or someone to drive a minibus to and from work placements).
Chris