I'm guessing that the Union believe they can make this work by providing the 7 days notice, then cabin crew turning up to work going through all the pre-flight and then deciding, just prior to boarding, to join their colleagues in solidarity. Probably on some rain-sodden island, in the middle of an A road...
Hence my thoughts on CAA legislation regarding mis-use of security identification.
I'm not sure, and I'm sure it's hazy from a legal point of view, what the mechanisms or procedures are for people taking industrial action who have already turned up for that day's work.
There are two very good reasons this wont work at BA. The first is that, by being a hub based airport, BA can just load standby VCC into T5 and should any crew use this strategy they can be replaced within literally minutes. The second is that it can't be used downstream because the moment they walk off the aircraft the crew are on their own; BA would have no obligation to pay their transport, hotac or even fly them home.
One of the reasons it works in the US is that Alaska operate out of several different gateways, and so it would be exceptionally difficult to locate any standby crew. Further to this, and as previously mentioned by another poster, Alaskan didn't have to give 7 days notice of their intentions.