What has just been delivered in the traditional English approach to such matters, it is not the only way of dealing with the problem. I find it somewhat unfortunate that often the reasons behind such decisions are applied as dogma. The argument that it should not be an offer (as with the Boots case) since the customer would not be able to change his mind should not lead to this conclusion since we can simply push back the acceptance of this offer to a later time (ie when the item is placed at the cash desk). The argument that such a system means that the seller is able to reserve the right to sell the goods in question could be dealt with by an implied term in the offer. This is not to say this is the correct approach -merely the issues raised in the cases do not automatically lead to the solution adopted by English law.
Another approach is adopted by the French law on such matters- they too deal with the same issues albeit from another angle. Under French Law and article in a shop window is considered an offer, however when an article is part of an artistic display the price of the object is not an offer; also if the price is not indicated it constitutes no more than an invitation to deal. As for the Boots Cash Chemist example, self service systems are considered an offer (note the famed Exploding Lemonade bottle case)- the contract being made when the item is taken off the shelf and placed in the receptacle, this does however require some form of implied term to allow a change of mind. One may suggest that this case was decided with the result in mind (the person was injured by the bottle and French contract law provided a better claim than tort, issues re cause etrangere etc).
This solution also creates problems- take for example a thief in the shop who places the object in their basket and only afterwards decides to steal it, if the contract is complete and property has passed there is no theft! Thus however can be dealt with by an implied reservation of title clause until payment.
I am not suggesting that French law is superior in this respect to English law, however, I am merely showing that the reasons outlined in the English caselaw do not necessarily lead to the English solution- the same problems can be adequately dealt with even if the display (whether in the window on or a shelf) is considered an offer rather than invitations to treat.