I did a UAL diploma which did prepare me well enough but I had a good few years between that and actually pursuing a degree. From my observations as a current art student, those who did the foundation year seem more well equipped to handle undergraduate in general. Especially if they stayed at the same university. The methods of learning and the marking rubrics are pretty different to most things people do prior eg. A-level. I sometimes wish I had done foundation just for that extra year of education.
Keep in mind that a lot of what you're paying for is access to resources - workshops, equipment, skills, print rooms, casting, woodwork, etc. This should rank as a top 3 priority when deciding where to go. If you desperately want to learn metalwork for example, it's useless going somewhere that doesn't do it.
If your end goal is illustration, then definitely go over the syllabus for courses. I do fine art and while it's pretty broad, art and design varies dramatically by institution. Some are 90% digital, focusing on things like video, VR & sound, while others are primarily traditional.
Regarding travel, I would avoid any longer than 40 minutes or so. Perhaps 1 hour if you're comfortable driving. You will be carrying heavy materials, sometimes unavoidably. Access and foundation courses usually cover at least a reasonable spread of disciplines so you may find yourself carting around a decent sized sculpture and wondering how the hell this happened.
Edit: Regarding portfolio, be prepared to show ideas, process. Completed paintings of clichés (eg. a portrait in a broken mirror or big hyperrealistic eye) might show skill in the discipline of representational art, but the majority of art courses today have a strong conceptual angle - even within illustration they'll expect you to demonstrate willingness to experiment and explore beyond a specialism.