In addition, the checks will create horrendous queues of lorries. I live near the Swiss-German border, and there are lorries queued up on on the motorway on Saturday waiting to get in to Switzerland on Monday.
As well as being tedious for the lorry drivers, the equivalent queues are likely to tie up a significant fraction of the lorry stock involved in cross-channel transport - and there aren't a huge number of spare lorries available to fill that gap. Those lorries in the queue obviously aren't actually shifting stuff - so the amount of stuff that can be imported will go down.
When there starts to be a little bit less of something in the shops, there will be panic buying, and yes, there will be bare supermarket shelves.
Obviously this can be sorted out over a few months, but April is looking like a really good month not to be in the UK!