BT's use of the term 'pre-register' creates an annoying lack of clarity in the contractual positionYes, it does! Isn't 1 of the components of making a contract is that both parties intend to enter a contractual arrangement?
There's four elements in a contract: offer, acceptance, consideration and intention to create legal relations.
Offer and acceptance we've already discussed. One party must make an offer they intend to be bound on acceptance by the other party, and the other party must accept that offer unconditionally.
Consideration is often a complex topic, especially when it comes to contractual amendments, but there's no real issue here. The basic formula is act for promise or promise for promise. The position that would be reached at eventual contract formation here is promise for promise: a customer taking up the free year's BT Privacy with Caller Display would promise to stay with BT for a year and BT would promise to provide a free year of Privacy with Caller Display.
Intention to create legal relations - which is what you were thinking of - is about whether the parties intended to create a legally binding agreement or not. It's most relevant in preventing the extension of enforceable contracts to most domestic or social agreements. If you promised your mother you'd see her for Sunday dinner if she'd cook, and she agreed, we've got offer, acceptance and consideration. What we don't have is an intention to create legal relations - you didn't intend it to be possible for your mother to seek damages from you in court if you failed to turn up for dinner on Sunday.
In commercial contexts, there is a strong and almost indefeasible presumption of intention to create legal relations. If you want to go Googling, the most relevant leading case is Esso Petroleum Ltd v Commissioners of Customs and Excise
 1 WLR 1. About the only rebuttal of contractual intent in a commercial context is an explicit one - an agreement describing itself as an "honourable pledge" was held to be non-contractual in Rose & Frank Co v Crompton Bros
 2 KB 261.
So, it's not intention to create legal relations that resolves this matter - it's in a commercial context, so the parties are taken as understanding that any agreement is legally binding unless there is very strong evidence to the contrary. The outstanding question is about what the parties agreed.
I took it that by pre-registering I was merely pre-registering an interest (just like those who pre-registered an interest in FTTC before it came to their area) and that I would hear from BT by email eventually (not as yet) with their definite offer at which time I could, if I accepted, actually register for it.
I took "Register to get BT Privacy with Caller Display free for 12 months from the date it launches* (a new 12-month line rental contract applies)", with the additional terms and conditions at the *, as immediate entry into a binding commitment (i.e. a contract) to take Privacy with Caller Display for one year from 6 December 2013, with a new 12 month minimum contract period running from that date.
Words take their normal meaning unless there is a good legal reason to depart from that meeting. If there's any dispute about the meaning of a word, the courts often turn to the Oxford English Dictionary. The OED defines pre-register as "to record or register in advance". That would be compatible with my understanding - you're registering for a service in advance of it starting.
I remember the various BT pre-registration schemes for various broadband technologies. The problem with your interpretation is that I believe you've read "pre-register" as "pre-register an interest" without any justification to insert the words "an interest".
Of course, it's not what I say that matters, but what a court would say. The test for the agreement is found in Smith v Hughes (1871) LR 6 QB 597
: irrespective of their real intentions, A is bound if they induce a reasonable person to believe they have accepted B's offer, and B entered into the contract believing A had accepted their offer. In that case, the dispute was over sale by sample: the buyer accepted the seller's sample of oats believing them to be the old oats he wanted, when the seller knew they were new oats. The first limb of the test is satisfied: a reasonable person would believe the buyer accepted the sample. The second limb of the test is also satisfied: the seller entered into the contract believing their sample of oats had been accepted and they would satisfy the contract by supplying oats consistent with that sample. The buyer's unilateral mistake in accepting new oats he was unable to use didn't excuse him from the contract.
In other words, it's not what you thought you were doing by accepting BT's offer that matters, but what an independent and unbiased observer would think you had done by accepting that offer. You accepted BT's offer believing it to be pre-registering an interest, and BT entered into whatever agreement has been created believing you'd accepted their terms. So, the issue is what that reasonable observer believes BT's offer to mean.
It really would have helped if BT had used clearer wording.