Staying at an hotel when under 18 in the UK?

My friends and I have to be in Cardiff for an event in May, however, there are no trains back after the event has finished until the following morning. We are struggling to find a hotel that will take us (for a moderate amount of money). Are there any UK hotel chains that will accept a group of under 18s? We were told that a youth hostel will take us but the concept is unappealing. Are there any other options which we could consider?