There is no legal obligation on you to keep the option open. If you have used the opportunity provided by home working to scale down your premises, the law accepts that the change may have to be permanent.
However, it may pay you to leave the option open indefinitely if you can. Even with employees who are long established as home workers, circumstances may change, and the reasons for opting for home work in the first place may cease to exist. If you cannot offer them the option to come back to work, they may decide to work elsewhere. You might be able to reach an acceptable compromise by offering a 'hot-desking' arrangement, or providing for the employee to come into work for two to three days a week.
