Rent Guarantee Insurance is available where all tenants have passed comprehensive referencing, or have had a guarantor do so in their place.
Unfortunately while overseas income can be included in the reference for your own info, only UK income counts towards the affordability ratio used to judge a pass/fail on the reference. As such it's unlikely that this guarantor would make your tenancy eligible for Rent Guarantee, even though adding them to the tenancy would clearly give you more security as a landlord.
Don't forego referencing due to a lack of UK income
Although not part of the affordability test, overseas income is included in the referencing report. So even if a tenant fails the affordability test based on a lack of UK income, the landlord is able to make an informed choice on whether they want to accept the tenant.
Tenants and guarantors might well be able to afford the property when their overseas income is taken into consideration. Landlords will also receive a wealth of other information from the referencing report, such as referencing from previous landlords and employers.
More information
Look at other titles on RGI and referencing:
You can see further information on the cover Rent Guarantee Insurance provides here.