Certain conditions need to be satisfied for the trivial benefits exemption to apply, these conditions are detailed at EIM21864
In particular the benefit can not provided in recognition of services performed by the employee, see EIM21868. Benefits provided in recognition of services provided, such as long service awards would not meet this condition.
As you are giving the flowers as a thank you for the work they have done during their employment this would not meet this condition and so the trivial benefit exemption would not apply. However, you may want to consider whether the gift meets the long-service award exemption.
The meal could be exempt under the trivial benefit rules, if it satisfies all of the trivial benefit conditions, as it is not being provided in recognition of the services of the staff who are attending the meal.