Companies House sincerely regrets the delay of announcing a test platform for the schemas due to come in effect on 18th November. I can confirm the testing service is now available.
The platform only currently offers XML schema validation.
Testing details
You can use your live presenter credentials - but show Package Reference as 0012 and Test Flag as 1.
Endpoint: https://xmlgw-sandpit-staging.companieshouse.gov.uk/v1-0/xmlgw/Gateway
If you receive an XML schema validation error, then the schema is failing due to a syntax error or incorrect basetype compatibility - and should be considered a failed test.
If you receive any other error response, such as an authorisation failure, then that means the schema has passed the schema validation stage, and should be considered a successful test.
For reference, the following is a link to the test service schema page.
SDN