An alternative - and I would say better - approach would be to enter the data directly into OSM, and then to make your own render of it, using the tools and styles of your preference. The data would then be available to anyone wanting to render or otherwise reuse, or improve it, along with that of neighbouring providers.

The important thing in that case would be to ensure that any updates made by South Central Ambulance Service NHS Foundation Trust (and doesn't that just trip off the tongue!) are mirrored in the OSM database. That could mean SCASNHSFT making the changes, or pushing the info to the OSM community or a willing third party. If only there were a mobile telephony company interested in facilitating such public service...