There is a lack of diversity within many UK Physics departments, be this diversity in ethnic backgrounds, gender, people with a disability and those from the LGBTQ+ communities, we are missing extraordinary talent and must work towards a more inclusive subject. The reasons for the current are complex and intersectional, and I refer you to the many articles published by the IoP for further details.
Fortunately there are things that everyone, but particularly schools and Universities, can do to work towards a more equitable Physics. I find the concept of "science capital" useful in understanding why some people choose to pursue science while other equally qualified people do not.
Despite being the "largest minority", there is still a lack of women working in physics. Below are details of some of the outreach events I have been part of to encourage girls to study science:
Marie Curious A day of talks and workshops for girls aged 11-14 who are interested in seeing what STEM has to offer. More information here
Big Questions in Astronomy Joint with the Museum for the History of Science, this (online in 2021) workshop is centred around a panel discussion with female researchers in Astronomy and Space Science. More information here
Oxford Women in Physics Society The society reaches 100s of members of the department through fortnightly events and a mentoring scheme (I was president from 2016-2018). More information here