I thought I should share some recent changes that we've had to make to our wp module since the worldpay upgrade to rbsworldpay.
In ub_worldpay.module we needed to make the following changes:
$test_server = 'https://secure-test.wp3.rbsworldpay.com/wcc/purchase';
$live_server = 'https://secure.wp3.rbsworldpay.com/wcc/purchase';
We could get payments through before but it's better to change to the updated url.