There is no hidden cheaper option, unless you get some kind of upgrade with a bundle, but even then, WiFi is the last thing they add for free. You can maybe wait until you get to port and use your roaming data, maybe with an EU roaming package. On a European cruise you will rarely have more than one full "sea day".
Also, be very careful about leaving your mobile data on when the ship sails. Sometimes, as soon as you get clear of land, the ship turns on a "Maritime Phone Services" antenna which is basically a satellite phone service, and it comes with data that costs a fortune (€10 a megabyte or something ludicrous). If that's the only signal it can see, your phone will roam to it. The EU makes providers send a warning at €40, and at €60 it blocks unless you explicitly lift it, but post-Brexit I don't know if this still applies to UK networks. And you can hit €40 in just a few minutes without actually using your phone because of all the background chatter that your apps are doing. (And for goodness sake don't answer the phone if it rings while you're on this service, because that costs several pounds per minute too.)
So always, always turn off mobile data before the ship sails. Better still, put the phone into airplane mode. (Don't just turn it off, though. I did that once, and then when I turned it back on when we hadn't quite docked yet, boom, "Welcome to Maritime Services" as soon as it went looking for a signal. So if you do turn it off, make sure it's in airplane mode first.)