"Prompted by the need for more space, Glock moved in that year to 1b London Road, Crayford, just over a mile up the road from the Jennings works on Dartford Road. The company remained in Crayford until 1991."
https://www.voxac30.org.uk/vox_ac30_cabinets.html
That page shows where the factory was - near to the greyhound track, which feels right.
So, the timing is right, and perhaps the place. However, the company were not just making the cabinets, but making combo amplifiers on site, which I would have needed to test.
I wondered if it was a smaller operation - perhaps inspired by the success of their neighbours, and with access to local amplifier experts.
However, I didn't work there because the employer told me that I had to take the role or lose my benefits. I then called my contact in the local job centre, who said that that particular company's YOP scheme had not been approved yet, and that they had jumped the gun. I was relieved not to have to work for them, and never contacted them again.
I think that, after 41 years, I have forgotten the detailed events.