You do not need absolute certainty before you apply, because a UCAS application is a shortlist rather than a final commitment. If you have space among your UCAS choices, applying to both universities you are torn between is a perfectly reasonable decision. The problem only becomes urgent when you have more possible universities than available choices, or when one of the options is there for weak reasons.
Treat your UCAS choices as a shortlist
Applying to a university does not mean you have chosen to go there. It means you want the chance to receive an offer. Your final decision comes later, after universities reply and you choose your firm and insurance choices.
This is helpful if you are stuck between two strong options where both courses suit you, both entry requirements are sensible. If you would be pleased to receive an offer from either, include both if you have room. You do not need to force a decision before UCAS if the application system gives you enough space to keep both possibilities open.
The harder situation is having six, seven or eight universities you like and only five UCAS choices. At that point, you need to remove the options you would not genuinely accept. Ask yourself a blunt question: if this were my only offer, would I be willing to study there? If the honest answer is no, it should not take one of your choices.
A university can look attractive for reasons that do not survive that question. You might like the city, the accommodation, the reputation, or the idea of being near friends. Those are real factors, but they do not rescue a course you do not want to study for three years.
Compare the course before the university brand
You apply to a course, not to a logo. When you cannot decide, go back to the course pages and compare what you would actually study.
Look at the compulsory modules, optional modules, assessment methods, placement opportunities, accreditation, contact time and entry requirements. These details are more useful than broad impressions such as it feels more prestigious or everyone says it is good.
Two History courses can lead to very different degrees. One is built around political history, early modern Britain and exams, while another gives you more space for migration, empire, social change, archive work and coursework. If those later topics are the reason you want to study History, the second course is the better fit even if the first university sits higher in a ranking
Assessment matters as well. If one course relies heavily on exams and another includes coursework, presentations and research projects, that affects your week-to-week life. Do not choose purely on the method you find easiest, but do notice where your strengths match the way the course is taught and assessed.
Entry requirements also deserve a practical check. A good UCAS list includes choices where your predicted grades make sense. One ambitious choice is fine if you would love to go there and you meet the stated requirements or are very close. A list made entirely of highly competitive options leaves you exposed. A list made entirely of courses you feel lukewarm about wastes the chance to aim higher.
Turn vague preferences into clear priorities
Indecision grows when every factor feels equally important. Write down the things that would actually affect your decision, then separate essentials from preferences.
Essentials are factors you cannot compromise on. That might include a specific accredited course, a commute you can afford, a campus with suitable disability support, or entry requirements that match your predicted grades. Preferences are factors that would be nice but should not control the decision on their own, such as a particular style of accommodation, a favourite city centre, or a campus that looked better on an open day.
Once you have that separation, compare each university against the essentials first. A university that fails an essential should leave the shortlist. A university that meets all essentials can then be judged on preferences.
Use evidence rather than mood. Open day impressions are helpful, but they are only one piece of evidence. A sunny day, a confident student ambassador or a new building can make a university feel better than it is for your course. Course structure, travel costs, support arrangements and realistic offer chances give you a firmer basis for deciding.
If you are choosing between universities that still look equal after this, choose the one you would be more disappointed not to apply to. That is a useful final test when both options are academically sensible and you have reached the limit of available UCAS choices.