I am starting in September too. I seriously considered the exact same thought as you. However, perhaps I have different ambitions.
I am particularly interested in strategy (War Studies KCL), and therefore, contemplated with the idea of transferring to KCL for my 2nd/3rd year. I even questioned whether to take an extra year if necessary. However, in terms of International Politics/Relations, Aberystwyth University is renowned and offers one of the best departments in the world. Depending on what you plan to do after you graduate, it is difficult for me to say without knowing your ambitions.
However, assuming you want to take IR/IP further after graduation, I would take most of the universities stated out of your list. I feel, LSE, KCL, St Andrews should be the only universities which you should consider if considering a transfer. Otherwise, I think you’d be demoting yourself.
I plan to take a Masters in War Studies at KCL, and having an IP background will only help me much more than without a single Strategic Studies degree.
The general reputation of Aberystwyth University is not that high. However, the International Politics department is renowned, and assuming you want to continue with IR/IP after you graduate, I do not believe there is any real reason why you should be so concerned on wanting to transfer. Furthermore, if you plan to take IP/IR further after graduation, it is probably recommended you take a masters in a subject related to IP/IR anyway.
I hope that helped.