I suspect the £1 million tag is either kite-flying or it has been thrown in to generate some publicity. If that were to be the hammer price you would have to factor in the buyer's premium plus VAT which might be anything from a quarter to a third on top!
Therefore, a bid of £650k to £750k would probably be enough to realise that figure.
But I'm surprised that the whole collection is being auctioned off as a job lot.
While selling the shirts individually may be time-consuming, auction houses do this kind of thing every week, but as others have pointed out, there are some shirts that are of low value and others that aren't.
I'm surprised they're not being auctioned as lots of two or three at a time with the star items being put up individually at the end to keep the interest alive.