How did the Mossad get the British passports?

According to the Times, the British authorities are now assuming that the British passports used by the Dubai killers were forged and are trying to work out how:

Authorities are investigating the possibility that British passport details were copied from the originals by immigration staff while the holders were travelling.

One problem: the olim whose names were used had lived in Israel for many years and were most likely Israeli citizens; in which case, according to Israeli law, they have to enter Israel on Israeli passports. So when exactly did the Israeli immigration staff have the opportunity to copy information from their British passports?

So, let's just say, this scenario seems unlikely.