I have had severe allergic rhinitis for decades. Among my arsenal of medications are pseudoephedrine/pseudoepinephrine (decongestant), oral antihistamines, oxymetazole nasal spray (ONLY ONCE DAILY, at night; using it more often WILL cause severe rebound congestion leading to addiction!), montelukast (prescription; leukotriene blocker) and aspirin (anti-inflammatory). I have also used the following types of nasal sprays: antihistamine, steroid (various brands) and cromolyn sodium. A saline nasal rinse (I use a ceramic neti pot and 1/2 tsp fine Kosher salt in 1 cup of distilled water) before using the nasal sprays clears some of the mucus and irritant/allergen particles from my sinuses. All the meds help somewhat, none of them very much alone; I rely on their additive effect. I live in the US; not all of these drugs may be available in the UK.
A caution about steroid sprays: they are associated with cataracts and glaucoma, so you must have regular eye exams while taking them. For most people they are highly effective, though.