The BBFC first classified The Terminator in November 1984. It was passed 18 without cuts, but not until after much consideration about whether the film could be cut to achieve a 15 rating.
When the film was submitted the distributor expected an 18 classification and therefore asked for any cuts advice that would help them to achieve a 15. Examiners viewing the film appreciated the appeal to 15 year olds, but were hesitant to suggest cuts to such a tightly edited film.
Some cuts were drafted, but ultimately the BBFC’s Director at the time, James Ferman, did not think the film should be cut and it was passed 18.
Here we see an Examiner's report which gives some of the flavour of the debate within the BBFC at the time. It also betrays its age. The BBFC has had published Classification Guidelines since 2000, and normally nowadays, with a borderline film, gives the distributor a choice between accepting the higher of the two classifications, or making any cuts which are necessary to secure the lower one.
When The Terminator was submitted on video in 1986 the 18 was upheld, though after the departure of James Ferman the video was submitted again in 2000, and passed 15 without cuts.