Re: The problem is not in removing the copy. It's with making the copy.
Correct.
That is an offence under the Computer Misuse Act of 1990 (as revised). Actually, several offences.
Section 1: Unauthorized access to computer material. Check.
Section 2: Unauthorised access with intent to commit or facilitate commission of further offences. Check.
Section 3a: Making, supplying or obtaining articles for use in offence under section 1, 3 or 3ZA. Possibly, if they wrote (or found) a script or other automation tool in order to gain access. Difficult to justify, even then, since they had only one target. But. you never know.
Section 1 gets you up to six months and/or a hefty fine. Section 2 gets you up to five years and/or a hefty fine. Section 3a gets you up to ten years and/or an unlimited fine.
So definitely not theft/stealing. No intent to permanently deprive, nor did they permanently deprive TalkTalk of anything. Breaching customers' privacy may be coverable by some privacy legislation somewhere, I dunno. Gaining credit card details with intent to defraud is definitely chargeable under fraud legislation and possibly the selling on of card details gains them a bonus conspiracy charge. Absolutely CMA offences. But not stealing.
IANAL.