Paraphrased from the Met dress code guidelines:
Ties/cravats must be worn with long sleeved shirts but when in "shirt sleeve order" they can be dispensed with unless when also wearing any item of outer clothing such as a pullover, tunic or anorak (the 'Metvest' is not classed as a garment) or a line manager directs a tie/cravat to be worn.
Staff may wear shirt sleeve order when working in a building. Elsewhere, a consistent approach should be encouraged. Line managers may stipulate whether
shirt sleeve order is to be worn by groups or individuals provided it is reasonable and practicable. Any clothing worn under the shirt must be of a colour to be
inconspicuous under uniform shirts.
When short sleeved shirts are worn open at the neck:
� Only the top button can be unfastened;
� Underclothing must not be visible at the neck;
� A tie or cravat must be worn if wearing an outer garment. During warm weather local management may exercise discretion on the wearing of a tie or cravat when there is a requirement to wear, over a shirt, a high visbility jacket.
http://police.homeoffice.gov.uk/publications/c ommunity-policing/dress_code.pdf?view=Binary