If you fall behind on your rent, you're in arrears. This is one of the situations where your landlord has the strongest legal position to seek possession of the property. Under Ground 8 of the Housing Act 1988 (as amended by the Renters' Rights Act), if you owe at least three months' rent at the time the Section 8 notice is served and at the time of the court hearing, the court must grant a possession order; there's no discretion.
For smaller arrears, the landlord can use discretionary grounds (Grounds 10 and 11), where the court can consider whether it's reasonable to grant possession. In many cases, the court may make a suspended possession order, allowing you to stay if you maintain rent payments and clear the arrears over time.
If you're struggling to pay rent, act early. Contact your landlord to discuss the situation, seek advice from Citizens Advice or Shelter, and check whether you're eligible for housing benefit or universal credit housing costs. Ignoring the problem makes it worse and reduces your options.