Why doesn't he trust me?

So I'm going to London in November for the theatre, and I want to do some shopping before the show. The train would take around two hours to get there. So let's say I leave at 10am and I arrive there at 12pm, that would leave me with around 6 hours to kill before the show starts.

I told this to my dad and he doesn't believe me. I think he would prefer me to take the train down to London closer to the shows starting time, then come home once it's over. He think I want to go down to London for the day so I can see my ex boyfriend for sex!! I have no intentions of doing that. Yes, I met up once with my ex boyfriend and that ended up with us being in bed together. My dad also remembers my ex and I talking about going to London together for university and so on, but that was nearly years ago!!

All the shops I want to go to are 30 mins/1 hour (walking distance) away from the theatre. I don't even know what university my ex boyfriend attends. I'm torn between lying to my dad and sending the ticket back and asking for a refund.
Why doesn't he trust me?
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