Do establish whether this is an education department with no clue as to the law, or a SS CP visit asap.
Copy the letter you sent to the school about deregistration to the Director of Children's services for your council and your MP.
Join Education Otherwise, and prepare your formal educational philosophy. Print out the "rules" on home education from the Education Otherwise site to have immediately to hand to refer to if they come back. Give them a copy of the rules and your educational philosophy when they leave with a huge smile.
(If the pressure is on for her to adhere to the NC, don't forget there are 3 online schools in the UK running formal 8 subject GCSE courses when she hits years 10 & 11, do remind them a teen suicide may not get as far as exams!). If she's volunteering the whole "socialisation" aspect is being taken care of and it's obvious you aren't hiding her out of sight doing something dodgy iykwim.
Tidy your house in case it IS a SS CP visit, just so everything looks OK if they turn up. If it is that sort of visit they'll ask to look inside your fridge (are you feeding the kid?) & at where the child sleeps - they have no right t enter your own bedroom.
It's very unusual for the police to be in attendance for anything other than a "OMG child at immediate risk of serious violence!" type call, simply due to the demands on their time - unless you have a criminal record for being very volatile and violent.
If it is, it may be instigated by your ex rather than the school - watch your back in this case and be VERY strict about adhering to any visitation agreements. Men that do this often don't realise that if a child is removed from the mother then that child may not end up in their sole custody but in a complete strangers care.
HV's are there to help with under 5's and it is not compulsory to have them involved in your family at all. If you do not find the one you are assigned supportive of your family life then you can ask for a different one, or just tell her that her services are no longer required. So long as you still take any sick kids to the GP's, have regular optical and dentists check ups etc noone can make anything of it. It's very common for Mum's to ditch the HV after the first, simply cos they don't feel they need any additional support for their second child onwards if everything is going OK.
As your child is no longer on the school roll, she is no longer their concern, so you do not have to tolerate any interfernce from them.
Stand strong & trust your gut - a happy thriving child is obvious to everyone.