I’m pretty certain it’s a Snap On.
Your only options here are really Snap On, Proto, or copies of these. Most of the copies I’ve seen have a feature that’s noticeably different from the Snap On, so I don’t think it’s one of those. Think it’s the Snap On not the Proto as the Proto has a bright pin, which the Snappy doesn’t.
The only ones that might have better availability in the U.K. than the U.S. are the Bahco. They do a couple of styles, but I don’t think it’s the Snap On style one, and the other is totally different.
If you can afford to even consider putting fuel in a Stalwart (they do about 3mpg if memory serves) you can probably afford the Snap On!