I'm not sure if UK high school prom's are the same as Prom's in America but I went to my prom and it was great! I remember planning to go about my anti-social ways back then and simply sit there for most of it talking to friends though this changed quite quickly and I remember being pulled up by a friend to dance. It was of course awkward at first until you noticed that pretty much no one cares if your dancing sucks or such xD Most people are not even paying attention to you when dancing.
Prom I'd say was actually one of the best nights of my life. Did a bunch of dancing, took lots of pictures with friends, and watched our teachers get drunk. One of my fondest memories of Prom was when everyone was sucking helium out of the balloons that were around just to make their voice sound funny. I even remember one of the science teachers saying we shouldn't do that but in about 5 minutes, he was doing it as well xD. Another fond memory was watching my teachers ride this insane bike that my friend brought to prom.. while they pretty much were drunk. (Have a look if you want: https://vimeo.com/236809356)
Anyway! To answer ya questions
I just went with a couple of my friends instead of taking a date and basically no one cares from what I experienced, most people at my Prom didn't even bring dates.
The only things I had to buy was my suit, aside from that I don't remember spending any money but you can buy some to buy drinks and food (Or at least we had too)
We had a professional DJ but I remember people going up to see him and helping him play and pick out music
I'd advise you go, it may be one of the best nights of your life! Brings some friends and just have fun, that's what it's all about.