I use the boots medium kit which was about £10 - £15 last time I checked.
Good kit with proper range of dressings, scissors, gloves, surgical tape etc etc, to that I add a military first field dressing, detol in little saches as it dilutes and goes a long way, a survival blanket, pain killers, zinc oxide tape maybe some compeed and some nail glue oh and a sharp knife.
Beyond that kit you are getting onto paramedics turf and I ain't a paramedic.