I don't believe Lexus specified a time period for the cambelt change in the UK. Just around every 60k miles (63k on 9,000 mile service schedule, 60k on 10,000 mile service schedule) up to the year 2000. Then it is every 100k miles.
If the belt breaks it doesn't damage the engine so no real need to replace early, maybe something like a water pump needed replacing so you might as well replace the belt at the same time, or the dealer recommended a belt change due to age even though it's not in the service schedule.