Attending an assessment centre while on a vac scheme

I begin a vac scheme with a mid size regional firm next week. This morning, I received an invite to an assessment centre for a training contract from a significantly larger international firm in London. The AC would take place 3 days into my vac scheme with the regional firm. How should I approach this? I’m leaning towards being upfront, but I’m unsure whether I should give minimal details or act as if I am hedging my bets for a post grad position. Alternatively, I just pull a sickie. Any advice?