These aren't hypothetical situations are they?

Yes, ownership normally passes to the customer once payment is made, but only because it normally says in the Ts & Cs that that's when the transfer will take place.

Well if your going to pay for the goods then does it matter if you eat them before you pay for them.
When you dine out you often eat a meal before paying for it.

Tricky to pay for grapes you've already eaten in the supermarket though. What do you give them to scan at the checkout?

Strictly speaking, yes, it's theft.

.... and a very common / chavvy / pikey thing to do

Eating the grapes is theft (assuming it is a bunch that needs weighing, as opposed to a fixed price packet), not the biscuits, until you (try to) leave without paying for them. So it is wrong (illegal ?) to use the bathroom until it is paid for. The point that it is in your house is irrelevant (even though possession is nine tenths of the law) so the Law is an ass.

"Strictly speaking" it is theft at the point at which you eat the biscuits as no contract (for their sale/purchase) yet exists. For a contract to exist you need offer and acceptance (and consideration but we'll skip that for now). Items on display in a supermarket don't constitute an offer, they are legally an 'invitation to treat', which sort of means they are inviting offers. When you take the biscuits to the checkout and offer to pay the displayed price you are making an offer to the supermarket and when they then take your money they are accepting your offer. It is only at that point that a contract exists so eating the biscuits prior to that point is theft (strictly speaking).

Although the phrase "possession is nine tenths of the law" is often used it has little or no legal relevance. Some argue that the correct phrase is "posession is nine points of the law" but that's also open to argument.

Getting back to the bathroom question, if the dispute only relates to the cost over and above the 'quoted' £4k then the customer really shouldn't withold payment for that amount. They should pay the £4k and then try to reach agreement on whether or not they are going to pay any additional sum.