This is just the general question at the moment but i prefer knowing stuff before it actually happens. What are the circumstances (if any) allowing the landlord to seek possession of their property during the AST?

The landlord must have a valid reason to seek posession of his/her property. These reasons are laid down by law and referrd to as 'grounds for posession. However, i believe that these grounds are limited in case of fixed term tenancies. Ground 8 is rather common: this is when the tenant falls behind with rent for 2 months or 8 weeks. There are other, but need to check these.

Am sure there are other grounds too, like tenant or landlord breaching the tenancy agreement, the tenant is causing persistant nuisance, damage to the property, etc.

i believe some grounds are whats called mandatory grounds where the court must order possession if this ground could be proved. There are also so called discretionary grounds where the court may order possession in some cases.

Guess rent arrears would be one of the mandatory ones. What would generally be considered a discretionary ground then?

As far as i know there are two mandatory grounds. In a nutshell, one of them is about rent arrears of more than two months and another one is about the lender seeking to sell the property (to pay off landlords rent arrears). The others are discretionary, for example tenant's responsible for damage to the property or the furniture, tenant is in breach of the tenancy agreement, delay in payin rent, etc.