Rachel Elnaugh became the first female investor on the UK television series Dragon's Den. She joined the original panel and brought experience from the business world to the programme's earliest investment negotiations.
Elnaugh was known as the founder of Red Letter Days, a company selling experience gifts. Her business background made her a recognisable member of the initial Dragon's Den panel, alongside investors including Peter Jones and Duncan Bannatyne.
Deborah Meaden is a frequent answer to this question because she became one of the programme's most prominent female dragons. She joined later, however. Sara Davies and Jenny Campbell were also later additions, so neither was the first woman to invest on the British programme.
The distinction concerns the first female investor on the UK series, not the first woman to appear as a contestant or the longest-serving female dragon. Those are separate records and should not be conflated.