From what I see, there is several teams that have good quality players and will annoy England a lot. Serbia is not the hardest team that England would face, yet the performance and the result are quite not that satisfying.
Group C is not that powerful and England will pass it relatively easily. The question is whether they will advance a lot or not and that depends from the team they will face. France, Germany and Spain are hardest teams in my opinion and the England team will struggle facing them.
Group C occupied by England should be considered the easiest group for England to face so the potential to become leaders in Group C is very open for England. Because it is true as you said that several teams that will make things difficult for England in this Euro are France, Germany and Spain. However, these three competitors will only meet after this group phase is finished because they are all in a different group to England, but I am sure they will all be in the round of 16 later.