I'd deal with the hotel directly tbh it's probably far easier. The number and email address are on the app.

From looking online its managed by Bespoke Hotels who typically don't manage any other Indigos in London. They do also manage the MGallery on Leicester Sq though which is an Accor hotel so I'm guessing their recommendation for dinner will be there.

I'll be surprised if they don't allow you to cancel without penalty or at the very least offer to swap the dates or a discount.