How many bed places (for how many people) are in the apartment?
All too often, when there are couples involved, the 'singles' often feel that the couple is taking over and it can become disheartening/sometimes embarrassing to the singles, resulting in people leaving. Now should the tenant leave, the licensees must also leave as they no longer have their landlord living there and they have not financial agreement with the landlord of the property.
Yes, they could live there as lodgers and you are incorrect that a lodger may, after 6 months request that they become tenants of the landlord and not a licensee of the tenant which the landlord cannot reasonable refuse. However, this only apples when one tenant of the tenancy has a Part 4 tenancy and not a fixed term tenancy. The lodger has the length of his residence in the property counted for his Part 4 rights. Thus, if he was a licensee during the last two months of a fixed term tenancy he only requires a further 4 months to acquire his Part 4 rights if he becomes a tenant of the Part 4 tenancy. There is also the disadvantage of the landlord perhaps not having a full reference of the lodger as and when he first resided in the property. Any agreement should be in writing.
If you feel that there is insufficient room in the property, although a two bed one (perhaps really only suitable for two adults and a small child, then you may reasonable refuse the licensees as tenants and in fact you could also use it for not accepting lodgers. You must also consider that 3 people living in the apartment are likely to cause more wear and tear than one person (who is the sole person on the lease agreement).