To be clear, a glow-worm is a kind of beetle. At this time of year, a glow-worm would be a larva. So you do at least agree it is a beetle larva of some kind. Given that distinguishing beetle larvae, even to genus level, is an expert task, so similar are they, the likelihood that it is a glow-worm larva, glow-worms being rather rare, and not another of the several hundred or thousand kinds of beetle inhabiting this country, is small. I took a quick glance at a picture of a glow-worm larva, and I would say yours isn't even the right shape.