This is a common problem for employers in Jersey, given the impact of fog or snow on flights to the Island. There have also been issues this year with strikes in France and the UK affecting ferries, trains and flights.
There is no obligation to pay employees who are unable to attend work due to issue with public transport. Each situation will need to be considered on its merits, but it may be that you can be flexible over how to manage the absence rather than simply requiring the employee to take leave.
In particular, if you offer agile working it may be that your employee is able to work from another location with no impact on their work.
If this is not possible, you could require the employee to make up the time later or take the time as paid annual leave.
We recommend that you deal with this in your contract of employment or handbook, so that both you and your employees know exactly what your policies are in case of any problems getting back to work after Christmas.