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.
Even though they were able to score goals and win the match against Serbia, we have to admit that if England performance was still not optimal and maximum in the match, Serbia was able to appear pressing and even often could trouble England defense in the match but fortunately England had Declan Rice and also a solid defensive line when they defended, Serbia who was not the favorite in the match in my opinion appeared quite impressive and even they Succeeding in pressuring England by creating more chances than England created in the match, England performance and play are still very far from good, but in the first match, of course it was a very natural thing.
Currently England is at the top of the group C standings, although they do have a better squad than other teams, but if they don't immediately improve their game performance in the next match, of course it will be very dangerous for England to maintain their chances of qualifying from the group phase, currently Denmark and Slovenia could trouble them later, I saw when the Denmark and Slovenia match was played, In fact, I was surprised by their ability that was good enough that it would most likely be able to trouble England later.