Since your sheet's locale is United Kingdom you can try the following in a new column (change ranges per your needs)
Now format this column as format > number > date.
If you need a more general formula use
Your spreadsheet is in a locale that currently uses the dd/mm/yyyy data format. The data has apparently been imported while the spreadsheet was set to a locale with another date format, which has caused the data to not get converted but stored as text strings instead.
When your csv data is in a date format that differs from that of the spreadsheet, it will ...