Because it's not that special that those deals fall through. I don't know about Dragons Den, but afaik Shark Tank US as well as the german version "Die Höhle der Löwen" had seasons with 60% of the "deals made on air" not happening.
It normally worked well for the start-up though as they used to get a lot of publicity/sales directly from appearing on the show and had found investment through other means.
This bit of advice has served me quite well over the years. At a celebration party for a new investment in a company I'd previously co-founded but was only a stockholder at the time, the CFO excitedly told me that I was 'worth millions', and asked 'how did it feel?'. I told him that "I'll tell you when we get an exit" and changed nothing about my life. Good thing, because while the check had cleared on that investment, the exit several years later wound up with the stock amount value down in the five figures.
One guy I know had co-founded a company, was doing OK. I noticed some impressive new equipment that showed up on his website and we chatted about doing some projects together and how he's got a major deal coming in and had invested well into six figures. The next thing I knew, his company had vanished and he was working for a small shop several states over. They didn't get the project after all, and were immediately bankrupt.
Worse yet, was a story from an attny friend I'd worked with; he told me about a guy (client?) he knew who 'got' a VC investment, divorced his wife, bought a big new house, car, etc. .... and the deal fell through.... not sure what happened to that guy, but I'm sure it wasn't pretty.
Repeat: "The deal is not done until the check clears"...
[edit: typos, clarity]
Probably not. I just assumed it was BBC news giving a bit of publicity to a BBC programme by regurgitating a public tweet.
It wasn't exactly top draw investigative journalism though looks like the journo did at least spark up the My Lupo website and add a couple of stock BBC photos. So that's nice.