We have a task (CORE-141) to add support for SagePay, but it is not on the dev schedule at this time. I'm not sure if the UK version is different, but if not, it would be available with the release of that gateway.
However, someone did integrate Omnipay into Blesta, which supports several gateways, including SagePay. You may want to check out that thread to see if it is a viable solution for you.