As Shoota has indicated, there has to be
intent "that someone will see them and be caused alarm or distress" in order for a successful prosecution for 'exposure' under Section 66 of the Sexual Offences Act 2003:
http://www.legislation.gov.uk/ukpga/2003/42/section/66
(The word 'indecent' was dropped when that statute was enacted).
Section 33 of Schedule 3 to that Act requires that a person convicted of such an offence be placed on the Sex Offenders Register if the following conditions apply:
"(a) where the offender was under 18, he is or has been sentenced in respect of the offence to imprisonment for a term of at least 12 months;
(b) in any other case—
(i) the victim was under 18, or
(ii) the offender, in respect of the offence or finding, is or has been—
(a) sentenced to a term of imprisonment,
(b) detained in a hospital, or
(c) made the subject of a community sentence of at least 12 months"
The relevant Sections of that Act (unlike some others) apply to the whole of the UK (so Sandy's title of 'Only in Northern Ireland' is far from accurate).
I'm surprised that some people here have assumed that the Sex Offenders Register applies only to paedophiles. It's likely that a high proportion of people on the Register have committed offences that relate to adults, rather than to children.