One thing that hasn't changed over the years is that the plod generally haven't got a clue about what specific legislation says is, or (by omission) is not, illegal.
To be guilty of indecent exposure a person has to intentionally exposes his genitals, and intend that someone will see them and be caused alarm or distress. [Section 66, Sexual Offences Act 2003]. Baring one's bottom is not an offence under that legislation.
The police might have tried to take action on the grounds of 'outraging public decency', which is a common law offence under which its illegal 'to do in public any act of a lewd, obscene or disgusting nature which outrages public decency'. However the courts have ruled that for conduct to be considered sufficient to cause such 'outrage' it must "go considerably beyond the susceptibilities of, or even shocking, reasonable people". [Lord Simon].
If the plod had bothered to check the law before acting, they could have saved totally wasting the time of six of their number.