My problem is how do you over come the VAT issue when you have a customer who is set up as an EU sale but you are shipping to UK.
VAT should be charged according to HMRC but when you try to post the sale it does not apply VAT even if you change the VAT code manually.
How can you overcome this without changing the default customer settings for his posting Group?
Try by changing Tax Area Code OR some field related to VAT Area Code. (this may be UK localization, which I am not familiar with, so the reply is just a wild guess).
Maybe the problem is not that you need to change the default settings for that posting group, but that a new posting group should be created to handle these types of customers.
Change the Sales Order Form to show the VAT Business Posting Group on the Order header.
is that OK?