If the child the paedophile has raped is not below the age of consent (e.g. because there isn't a minimum age of consent), then a rape charge won't be automatic - i.e. it would have to be proved that the child had not consented, not just that intercourse had taken place for charges of rape to be brought successfully. Of course, a rape charge should/would be possible, but it wouldn't be automatic. Thereby making life easier for paedophile rapists.
Actually under the Sexual Offences Act 2003 it is illegal for a British national to travel abroad to carry out an act that would be an offence here, so s/he could still be prosecuted. CEOP actively pursue travelling sex offenders abroad and no doubt will be very interested to hear about this book.
Also to address an earlier point, the UN Convention on the Rights of the Child regards a human being as a child up until its 18 birthday.