The shareholder discount is excellent value. At present prices it costs about £440 to purchase the required 200 shares. So to get the value from the purchase you need to spend at least £4400 over the course of your ownership to make it pay.
You must be registered on 1 November to be able to book discounted travel from 1 January.
You get 10% off all exUK bookings, for you and up to 5 companions on the same flight. Must be on BA metal. Must be exUK. No code shares.
It applies to the fare component only, so not applicable to the taxes and fees element (which can be considerable).
You still earn miles and tier points as normal, though these fares are not upgradeable using BA Miles, and do not accrue OnBusiness points.
More information can be read at http://www.bashares.com though I would hold back until midsummer when a combination of the oil price and the inevitable collapse of the T4 baggage system will cause a further drop.
The shares will likely then rise as the benefits of a working T5 flow through, fares can be raised and BA once again becomes a takeover target or merges with AA (personally I think this is what has been sidetracking Willie Walsh from T5 oversight).
To answer your specific question, unfortunately fares do move during the booking process and can sometimes become unavailable, just as with concert tickets. Technically, you have no contract until your payment has been accepted.
In this case you were not able to complete payment, and so had not contract. Aside from some possible IT issues, BA has no duty of care in such a situation.
This can be a particular problem if you are not familiar with the booking process and require extra time to complete the transaction.