This very point is discussed and answered here:
"But offenders who are prosecuted abroad and are deported back here after serving their sentence are still not required to sign the sex offender's register. They are free to travel and abuse children abroad again. The Ecpat report refers to a 77-year-old British offender who has been arrested six times in Thailand since 1991, for various serious offences. Yet despite this, he was free to travel back there again in March, when he was arrested for abusing an eight-year-old boy.
Gadd/Glitter has a previous offence of child pornography so he must re-sign the sex offender's register, and we are assured "will face up to five years in jail if he does keep to the rules". The reality is somewhat different; nearly all non-compliers are simply fined, often less than �100."
http://www.guardian.co.uk/commentisfree/2008/a ug/22/ukcrime.childprotection?gusrc=rss&feed=u knews
In a nutshell, it is because he has already been convicted in the UK for child sex offences