starting a new job dilemma

Ok so I've got a part-time summer job lined up for the start of July (which is like a week away). My dilemma is how I'm going to get around missing the first 2 days without looking like an absolute idiot.

For context - my birthday is this week and as a gift, my friend got us concert tickets which aligns with my first day of work. What also doesn't help is we live like 3 hours away from London so we are going to stay the night as it will be impossible to get public transport so late. To conclude the timing could not be more off.

I know one of my coworkers did something similar at my old job, she communicated that she was going abroad and would start as soon as she got back but I think she did it well in advance and told them in the interview whereas my 2-day trip popped up very last minute.

The job itself is a temp part-time summer role as a store assistant, nothing too demanding however I know they're going to have to find someone to cover the 2 days which reflects so poorly on me.

Yeah so how should I go about this, should I call in beforehand since I have a week to notify them? should I just pop by at the induction and tell them a day in advance? my friend said to say its a family emergency but I feel guilty lying about something so random.