Hello,
If all your customers are in United Kingdom, you can see to unpublish all other countries and check that your address country field is right set to use UK as default value.
In the SagePay plugin, you can still activate the "debug mode" in order to store some debug trace in the HikaShop payment log ; there you will be able to see what the plugin send to SagePay and what is received.
Best would be also to check your orders in your backend to verify if the customer addresses are right set (for the orders with issues).
Sometimes, when there are some "reading" issues, the message returned by their platform is inaccurate.
So it's might not the country field which is problematic but another field which generate an issue in their side and the first error their platform see is a missing country (but the problem is that they can't decode the full content correctly).
We saw some kind of issues regarding the "basket XML".
For the rest regarding SagePay, we have other threads talking about the situation on the plugin :
www.hikashop.com/forum/checkout/888632-o...-multiple-times.html
www.hikashop.com/forum/payment-methods/8...still-supported.html
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.