I was not aware the share codes last that long now, the last time I used one (for work, although this was several years ago) it was only valid for 4 days and my employer refused to accept it which was a whole thing. I was under the (incorrect) impression that the system for border clearance would be worse than that. So I have changed my mind on this point despite your flippant comment, which was argumentative for no reason.
On the case of trusting the UK government to manage the immigration status digitally for people I still have my doubts. They don’t have the best track record.
I have my own UK passport now so I am a bit out of the loop but any sort of system where a person doesn’t have offline proof of their status that they can keep seems like it is open to issues (although physical only also has its issues so there are pros and cons no matter how you approach it).
As others have mentioned, core exercises will be key here.
Some more traditional non-core exercises like squats and deadlifts will be engaging your core muscles (and a reminder that your ‘core’ is your whole trunk/body and not just your abs!), but you can progress this along with additional core exercises.
As someone that sits a lot for work, I really like to do ‘supermans’ and variations of them, a basic one is to lie flat on your stomach with arms and legs extended, then raise your arms and legs off the floor. You can hold, do them as reps, “swim” in the air, add in some pulls (like an overhead row if you have a resistance band or something you can attach in front of you).
Yoga has been mentioned below, but I would add Pilates to the list as well, there are plenty of core exercises to go at.