If you're looking for a UK Service provider, we have UC320's on our network, have open support tickets with Cisco TAC on behalf of customers and understand the product pretty well. Don't hesitate to contact us.
I'd disagree with "This is rather a vital approval considering that the BT_IPVS is where Small Businesses in the UK are going to be connecting to.". There are any number of UK SIP carriers. And two approved/validated ones for use with the UC300:
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