Most important message is if you think your child is very unwell, please take them to A+E. Some concern, as with adult emergency services, that people are leaving it too late.
Absolutely! Fever in a kid that lasts for more than 5 days always needs checking out by a professional, now is no different to before.
Same with any other symptom that causes a parent to feel particularly concerned, breathlessness, lethargy, not eating or drinking, abdominal pain and bloating, rash etc.
Go straight to paediatric A&E for blood tests (GP will only send you there anyway).
Check online that your normal A&E is operating before you leave (we normally have a choice of two at the same distance, but one closed to increase their CV capacity, redirecting A&E kids to the bigger one).