In some cases you may be asked by an employer to treat a benefit
as exempt from tax, on the grounds that the cash equivalent
(Section 203(1) ITEPA 2003) of the benefit taxable on the employee
(or on each of the employees concerned if there is more than one of
them) is so trivial as to be not worth pursuing. This is sometimes
referred to as 'de minimis' grounds.
There is no general statutory limit below which benefits are not taxable. However that does not mean that you should insist that every trivial benefit should be included on a form P11D or included in a PAYE Settlement Agreement (PSA), irrespective of the administrative burdens on both the employer and the Inland Revenue in handling P11Ds and PSAs.
When considering requests use common sense, bearing in mind the guidance at EIM21861. Strike a balance between sensible practical administration of the tax system and the need to deter employers from providing what is in reality part of the remuneration of their employees in a form that seeks to exploit that practical administration.
Do not discriminate against large employers by seeking tax in circumstances where you would not if only a few employees were involved.
See EIM21862 for what to do if you agree to the employer's request.