I am not on the defensive at all, and never worry about proving a point I like a good debate and try to and answer questions and points as fully as possible ! That way we can all benefit and hopefully learn from others experience.
As far as VAT is concerned you are correct about simplified invoices for transactions under £250, and having read the HMRC website vat-invoices.htm#3 I can see where you are coming from so I have learnt something from this that I can use in future so thank you for that.
The issue I can see from this though is how do you deal with overseas customers, where is a site is likely to have significant overseas sales, if you are entering prices with vat included into the database (I encourage my clients to enter a net price and then I handle this with a function to calculate vat), the most recent site I have built www.tshirt-town.com (this is an eCart site not a powerstore site btw) sells a lot of stuff overseas and by having net prices and a "proper" invoice I can handle the non vat sales easily. I know this has gone off on a bit of a tangent, call it clarifying my position :-)