Definitely call and ask about the possibility of rescheduling. Be sure to tell them you've already made plans to attend Develop during that time -- In fact, say 'the Develop Brigthon game dev conference" because you may well be speaking to a receptionist or personal assistant-type, and you want to make sure its not lost in translation that this is something you're doing to grow professionally, and not just turning them down for a day at the beach.
In actuality, it might even be that they're sending staff to the conference themselves to present or scout talent. It's not at all uncommon that they might conduct interviews with individuals at the conference. You might even ask if they plan to do this, and if you could get an interview slot there. Keep in mind though, that those kinds of interviews are brief and are more of a 'lets get this guy off the show floor and see if he's someone we want to interview seriously' kind of thing, and if they're already bringing you in for a deeper interview you may only stand to lose out by 'interviewing' with them at the show, so consider your options carefully if they give you the choice to reschedule or to interview at the show.
If they do have staff at the show, *and* you ran reschedule your on-premises interview for after the convention, do make it a point to meet and introduce yourself to them. Just politely approach them and say "Hi I'm so-and-so, I'm glad to see you here, I'm interviewing with your company next week." and see where the conversation takes you. It never hurts to make your face known, and being at the show will probably even earn you a brownie-point or two -- it certainly won't hurt your case as long as you don't overstay your welcome with them.
If they absolutely can't reschedule and don't have staff at the conference you can interview with, then you might have to miss the conference or eat the cost of an adjusted train ticket, but its highly likely they'll be accommodating and you can cross that bridge when you get there.
Don't sweat any of it though. I see no downsides here.