I would disagree with one part of that UK Care Guide advice.
…if your girlfriend lives with you permanently, you should notify your landlord and add her to your lease.
New relationships are often fragile and not always forever, particularly amongst young people. The last thing a tenant needs is to be forced into a legal entanglement with a new girlfriend or boyfriend where they become legally responsible for each others rent debts. The tenant would also be more likely to lose his/her home if they split up. I have always suggested that permission from the landlord is sought for the person to be a permitted occupier at least for the first year or so, if the landlords insurance and mortgage conditions allow this.
Permitted occupiers do not need to sign the tenancy agreement but need to be on the tenancy agreement so that the tenants sign and agree that they are permitted occupiers.
That’s not necessary. It can be done via a side letter. The landlord sends permission to the tenant to have person x as a permitted occupier at the property at their discretion for a period not exceeding the length of their tenancy or period of occupation, whichever is shorter. The PO would not acquire any tenancy rights unless they marry and cannot pay any of the rent to the landlord.
It’s likely to depend on when the PO occupies the property. For example, I have one tenancy where the PO is the tenant’s spouse and is named on the TA as a PO but not a signatory to the agreement. This is because she is a dependent of his and he is on a study visa.
In another property, I have a PO who is the later partner of a tenant who is on the TA but who had occupied the property for 5 years before the PO moved in. In this case, I used a side letter as David has mentioned.
I assume that’s true as long as he maintains his right to rent. If he no longer does, e.g. because he fails to apply or is unsuccessful in his application for a graduate visa, then because her visa is dependent on his, I would assume she would forfeit any right.