Absolutely the take up will be high, very high - but it will depend on the dispute. Let's think about this.
Currently:
RMT vs Network Rail - lack of signallers are shutting down the railway, but the dispute isn't about their situation and on this forum plenty of signallers have complained about RMT grouping them into this.
RMT vs TOCs - check out a random TOC (GWR) -
https://www.rmt.org.uk/about/ballot-results/defend-jobs-pay-and-conditions-gwr161122/?preview=true - 30% did not vote, and even of those that did vote, 10% did not want to strike ('are you prepared to strike ' is the question)
ASLEF vs TOCs - would be surprised if there isn't a driver or two needing cash for something....
.. but in any event, the union will be _obliged_ to pick staff for the job, randomly or otherwise..