"The delay was caused by Network Rail / the infrastructure manager / a third party"
high likelihoodThe operator points at signalling failure, a broken-down freight train, trespass or someone else’s infrastructure and says the delay was not their doing.
What answers it
Under the contractual UK Delay Repay schemes, cause is irrelevant to your entitlement: the schemes pay on the delay, whoever caused it, and the operator recovers internally from the infrastructure manager. Under Regulation (EU) 2021/782 the railway undertaking is the passenger’s counterparty and only a genuinely exceptional, unavoidable circumstance excuses it — and a staff strike at the railway itself is expressly not one.