You can claim regardless of strike action or not. (see below)
I used to have more tube vouchers than real money at one point. one advantage of living end of the line i suppose.
Customer Charter refund pledge
If you experience a delay to your journey of more than 15 minutes, for reasons within our control, we will refund you with a voucher to the value of the single delayed journey. Please note that refunds cannot be made if a delay is beyond our control or that of our contractors such as security alerts, third party action or adverse weather conditions. This also applies to service changes advertised in advance.