shipping Strange shipping behaviour on Prestashop

[d3lvedesign]

I have a strange issue with Prestashop v1.6.1.5 on a client website that was setup early last year.

It seems that some customers are able to check out using International shipping even though they have setup and saved a UK address.  A client has sent me some orders which clearly show that the customer has a UK address but changed shipping type to International and when checking out was able to get FREE SHIPPING because International shipping is set to FREE.

I've advised the client to set a price for international shipping but because they don't have a set rate from their courier, they require the customer to call them to get a quote after the customer has purchased an item.  This is why international shipping is set to free. 

What i don't understand is how some UK customers are able to even choose international shipping if they already have a UK address saved upon checking out?

I have logged in and tried it myself, but i can't workout how you can select international shipping after setting your address as UK based.  The only way you can do it is by adding a new (international) address and setting as default, but then it only provides you with international options.   

Has anyone else experienced this problem?

Any help would be appreciated.

(Screen shot of order confirmation is attached)

Edited January 29, 2018 by d3lvedesign (see edit history)