Level 3

Supplier not registered for VAT - which VAT code?

Hi all
30 years a photographer, just gone over to QBO after two decades on desktop.
Qs x6:
Now we have MTD I am going to have to be careful with my supplier VAT codes. I’ve not usually cared if I used E, Z or N as it made no difference, it just had to be 0%. But now that the HMRC are going to be sniffing around the intimate details...
I know N code is for a not registered supplier, like my freelance assistants.  But...
1) what about the Post Office, or
2) British Airways, or
3) my Insurers? Are they Zero rated or Exempt? How do I find out?
4) And what about a lost receipt, when VAT has been paid, but I cannot reclaim it because I have no proof of the rates and amounts, what code for that?
5) Also, if a receipt is for a small sum (what is the maximum?) and the VAT rate is not identified on the (say) till receipt, and the net and VAT amounts are not broken down, BUT the supplier has printed their VAT number, what do I do when I can safely assume VAT has been charged, and should by rights be reclaimed?  Do I now have to ask for a full VAT invoice for every cup of tea and buttered scone I buy while out on location? 
Thanks in advance
John Wildgoose