Even if your tenants fail referencing, RGI will still be available if they can provide a UK-based guarantor that passes Comprehensive referencing in their place. 

For tenancies with multiple tenants, every tenant's share of the rent will need to be covered either by a tenant or guarantor(s) who passed referencing.

Multiple guarantors can be added to the application to cover any shortfalls in rent share affordability. 

Where there is only one tenant then the guarantor must be able to cover the entire monthly rental amount.

You can purchase guarantor referencing here:

If the tenant is unable to provide a UK-based guarantor, RGI will not be available for your tenancy. 

You can find more information how to proceed when a tenant has failed referencing here:

Can I let to a tenant who fails referencing?