Can Landlord Prevent Tenant Becoming a Foster Carer?

Landlord wants us to write to tenant to stop her becoming a foster carer. My thoughts are this is discriminatory and not something the landlord could reasonably enforce (apart from issuing notice requiring possession) and risks legal action if they tried?

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Sub-letting/Assigning Tenancies

A landlord who has taken care to select a tenant by proper referencing and verification of suitability is unlikely to allow that chosen tenant to sublet, assign or transfer the tenancy to another without the landlord’s permission. In the past, tenancy agreements always tended to prohibit subletting or assignment.