Yes, if the net invoice is less than £250, you can utilise a retailer's VAT invoice and only add VAT to the total. But for anything over £250, you have to figure it out line by line. Good if you don't sell much! You can use vatcalc.dev to double-check changes.
Yes.. that's right. The only real choice is to store VAT per line, especially when there are different VAT rates. It's clean when you match what the provider actually charged per line. It might be possible to use the same form for both sales and purchases, but it probably would not be worth the...