When a bag search is conducted no items are supposed to be opened the retailer is supposed to only look at the contents. It's typically somewhere near entry and the check-out. It's supposed to be prominent, but that is subjective. bags from other stores), or other items (people often shoplift and store small but valuable stolen merchandise in a cup with a lid and straw) they can typically (with signage) search anything that may potentially be used to shoplift, however, retailers must display a notice that your entry subjects you to this right. So long as the store has some sort of notice, retailers are able to search any bags, parcels (i.e. When a customer enters a store, they do so under a licence. Many stores, especially in urban areas or stores that are very busy, will have a notice or some sort of sign that says something to the effect of "we reserve the right to have you check (stow) your bag while shopping, or to search your bag before you leave". In most stores it's quasi-contractual in that you enter the store as a "licensee" subject to the terms of that license. If they detain you against your will and you are later found not to have shoplifted, you may be entitled to bring a claim for the tort of false imprisonment, which is, again, actionable per se (you need not actually show damages) and serves to vindicate a person's right to liberty.
You are under no obligation to remain in the store. In any case, they do not have the power to arrest or detain you unless they believe you have committed a crime, and in those circumstances, only reasonable force may be used. You need only prove that these occurred, without actual loss or damage. If they attempt to force you to surrender your bags for search by physical force or by intimidation, you may be entitled to bring a claim for the tort of assault and/or battery. It is for this reason that you will often be asked by security staff to open your bag, and move belongings around inside that may obstruct their view. In Australia, bag searches must be consensual - shopkeepers and even security staff have no power to search your person or belongings. I can't find any specific laws or cases in the United Kingdom.