Zen have a couple of weeks to sort out this bandwidth allocation otherwise I'm cancelling my DD and off elsewhere as the product no longer meets my needs.
And good luck to Zen in enforcing the fixed term contract in the small claims court as only 50% of the advertised product is being supplied.
I would expect you to lose any legal action.
You entered into a binding contract with Zen on certain terms, including a set monthly bandwidth allowance. A piece in the newsletter about changes to monthly bandwidth caps does not constitute a contract to amend the original contract between you and Zen, as you provided no consideration for it (you neither paid a price nor made a new promise) and it was not executed as a deed. Moreover, you cannot invoke the doctrine of promissory estoppel, as you did not give up a legal right in connection with the newsletter article. There is therefore no way for you to rely on the newsletter article in any action for breach of contract.
You may be able to succeed in an ASA complaint - I can't say whether the ASA would accept Zen's explanation that "broadband" does not include "fibre broadband" unless explicitly stated as such. Even if you succeeded in an ASA complaint, it would not change the contractual position.
If you cancel your Direct Debit, you lay yourself open to legal action by Zen and their agents.
Ultimately, Zen are entitled to rely on the original contracts with fibre customers on the old products.
There is not an obvious equitable way to permit upgrades by those still in minimum term contracts. Some paid installation in return for a 12 month minimum term, others committed themselves to a 24 month minimum term in return for free installation. Those on the legacy 400+ GBytes/month packages are on a wholesale product with enhanced priority, which has a minimum term and is not used in the new product line-up. How do you propose to deal with these problems fairly?
Each existing customer freely entered into a contract with Zen. So long as Zen continue to provide the contracted service for the minimum contract period, they are entitled to rely on the contracts. Though it is often possible to upgrade early to newer tariffs on telecommunications products, there is no right to do so.
To be clear, I would rather Zen found a way to allow early upgrades, but they have decided early upgrades are not available in this case without a termination fee that I understand would be the entire sum due for the remaining minimum contract period.
Long term customers have been used to dealing with Zen on a 1 month minimum contract, where, as you pay a month in advance, you are out of your unpaid minimum contract period immediately. This was not the case on the fibre products, and customers cannot expect to deal with Zen on the same flexible basis as when a 1 month minimum contract period applies.