I woud also be wary of using too many sporty people. They have a talent that has been coached outside school. Their success is often not down to the school at all. It is certainly not down to brilliant teaching. Although I do accept they are dedicated.
In addition, the notion that: "you can be what you want to be" is totally meaningless if the young person does not understand how to get exam grades and put themselves in the best possible light to potential university/college tutors and employers. If they just think they can rock up to university or employment with the minimum of effort, they are wrong. Young people are better advised to be realistic and work for what they want. Very many people cannot be what they want to be because they do not understand how to get there. It is your job to focus on what they need to do and not just list people who are super talented or who have influential parents.
I am never quite sure what "role models" actually bring to the party either. They are usually people who are very bright, very talented or, as described above, have wealth and influential parents. Role models only inspire the really talented who may well have got there anyway. Therefore you may not grab the attention of the majority with this list. Much as we all love Jesica Ennis Hill, there is only one of her out of 66m people in this country. The rest of us do not appear to be inspired to emulate her - because we can't! Ed Milliband - he was not well connected then? His whole upbringing was surrounded by influential politicians and political debate in his home. He never went into the real world of work either. No, do not mention him!! Kate Moss may not be a very bright suggestion either - she took cocaine didn't she? That says you can do drugs and still earn millions. She also has the money to go into rehab. Ordinary people do not and turn to a life of crime to get the drugs. Her modeling potential was spotted, but had nothing to do with school or exam results!
I would try and illustrate the talk with young people who may have been refugees, have achieved well in business, become well qualified through hard work and have achieved a lot from a disadvantaged start. What skills did they hone? What attributes do they have? How did they overcome adversity?
I doubt very much that the institution you are going into - unless it is a young offenders' institution (which it is not) - would recognise itself from the way you describe it. I might be pretty cross if I was the Head that you thought it was such a dump. I might want to get to know it better before you roll up and make a patronising speech. What does their latest Ofsted report say, for example? So many schools have improved, this school may no longer be what you think it is.
I would also get the name of Rafe Spall's school correct, should you use it.