Raising the rent the lawful way: Form 4A, notice periods, and the tribunal
With fixed terms gone for new tenancies, the statutory notice route is how rent moves. Get the form and the notice period right and it's routine; get either wrong and the increase simply isn't binding.
The mechanism
You propose the new rent on the prescribed form (Form 4A for assured tenancies), giving at least the statutory notice period before it takes effect. One increase per year is the general shape of the regime; check the current form and period on GOV.UK before serving, as both are set by regulation and can change.
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If the tenant challenges
A tenant who thinks the proposed rent is above market can refer it to the First-tier Tribunal before the increase date. The tribunal sets the rent it considers the property would command, which can be lower than you proposed. A non-compliant increase (wrong form, short notice) is not binding at all.
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The practical rhythm
Diary the earliest next-increase date when you serve one, keep a copy of the served form with its date, and never rely on an emailed "rent goes up next month" — the form IS the increase.