It's not theft until she leaves the store without paying for it as it's impossible to prove dishonesty (as Raymond has pointed out a number of times but people are still blindly arguing whilst ignoring the law)
The point that it may be a store which charges per kg is moot, I'm sure Tesco are more than capable of charging someone for a unknown sized banana
People are either looking at this from the wrong angle or thinking too deep.
Store policy regarding eating before purchase - irrelevant, policy is not above the law
Personal opinion on whether it is a chavy thing to do, or a disgusting habit - irrelevant.
Personal opinion on whether it is theft - unfounded, the test for theft is not a personal opinion but comes down to the test for dishonesty
As to the matter of whether it is weight or sold by unit - irrelevant, the test comes down to dishoensty. If you can't prove she never intends to pay for it then she is not guilty. How the price is measured is a whole other subject. If she has a trolley load of food, and a wallet full of cash then it would be hard to argue, over and beyond reasonable doubt that she can't afford to pay for a single banana. Push comes to shove, she can offer £10 for it, who is gonig to argue that her banana cost more than that in weight?
Don't need to think of anything else. The only question you have to ask and satisfy yourself with is:-
Can you prove she never intend to pay for the banana.
I can't, not until she walks out of the door. If she paid for the trolley full of food and didn't declare the banana then yes, stop her then, and you will have your grounds and have your mens rea.