D323 is not controlled by Swanwick Mil but normally by Boulmer or Scampton.
The only area where Swanwick Mil offer a service (unless by prior agreement) is the Pennine Radar area, detailed in the AIP. It states that service is subject to unit workload, so the inference is that service provision is not guaranteed. Operators should be prepared for that. Having said that, if a long term service reduction is planned, then a NOTAM from Swanwick Mil would be sensible.
For other airspace, the company must have an agreement with Swanwick Mil for service provision. Eastflight are one such company that spring to mind for operations North of NATEB. In my experience, Swanwick Mil are very good at keeping such companies in the loop and giving them the chance to decide whether to refile through Controlled Airspace or take their chances in Class G.
Which airspace are you referring to ?