If you think it would be worth a try, then go for it. I know you've said not to say you'll probably break up eventually, but you have to accept that it is a possibility, and if it did happen, you don't want to regret choosing your university just to stay near to your boyfriend. Your relationship may not last forever, but once you have your degree you will have it forever, and it's important that you choose the one that's right for you and that you think you'll do well in. If your relationship is going to work out in the long term, you'll be able to make it work across the distance, and if it isn't, then it won't work out regardless of how close you are. You just have to make sure you communicate with each other and trust each other in a long distance relationship. It will be difficult, and you will miss each other, but you'll easily find ways to keep yourself busy at uni and pass the time between visits.
Don't let him put you under pressure to choose Cardiff just to be close to him. You following your dreams shouldn't be something that affects your relationship badly - he should be supportive of you doing what you want to do. If you did go away, you'd be away for 3-4 years I'm guessing, which isn't a long time if you're going to be together in the long term. An hour away is really not far, that's the distance currently between me & my boyfriend (although he isn't studying anymore) and we manage to see each other pretty much every weekend. Yeah I really miss him during the week, but the time together feels a lot more precious and special than it did before, when he was at uni with me, and the time flies by between visits. Saying goodbye is hard but it always has been for me, even when he was 20 minutes' walk away!
So my advice is: choose the university that you want to go to the most, and if your relationship is meant to work out it will. LDRs are hard but definitely well worth attempting if you love the person but if you feel it's getting too difficult and you're finding missing him overwhelming and it's affecting your studies, you need to think about whether you are happy to be in the relationship.