I have never heard of anyone losing pay or seniority due to being on strike. I would think that the unions would make quite a lot of noise if it happened.
Hopefully, someone who can give you details of any rules or legislation will come along shortly.
On a serious note, there are laws protecting striking employees if their union has followed the proper procedure; in reality an employer can lawfully sack an employee following strike action if he follows the proper procedures.
Striking employees can expect to return to work under the same contract, with the new terms if the strikers and employer came to an agreement.
What's more likely is the Royal mail will lose alot of buisness / customers and this will mean less staff needed and less money to afford payrises next year
RM is still the only company that charges the same to deliver a letter to your next door neighbour in London as to the Scottish highlands and islands, I don't think they'll lose business.
Reliability is a factor though. Also habit...I use to send smallparxels royal mail but have recently been impressed by fast cheaper service from couriers so won't go back... and alot of people has decided to use this year as a reason to stop sending dozens of Xmas cards each december.
And for letters I try to do by email WhatsApp etc.
Mail will decline alot next year
Oddly enough, the last two items I have bought from Amazon (in the last couple of weeks) have been delivered by RM. I bought them from Amazon, not marketplace sellers.