Is it possible for me to revert iOS versions from 15.4 to 15.2 on an iPhone 8?

No, Apple is no longer signing the earlier versions of its operating systems. Not only that, reverting to earlier versions would mean not having patches for bugs and exploits.. you’re always better off on the latest possible version.

Okay, so it seems that Apple are trying to force us to purchase a newer phone. My iPhone 8+ was working perfectly until I installed the latest updated IOS 15.4.1 (a couple of hours ago) - MY iPhone 8+ was previously running IOS 15.2.1 without issue. After installing the update, and restarting the phone, it is now constantly searching for network. I've used the sim in my old iPhone 8, and it works perfectly, yet when replaced in the 8+ still the same issue, unable to find the network. By the way, this isn't a network issue as I have 2 sims (1 for work and 1 for personal using different networks) neither sim works in my 8+ anymore, and yet they both used to work in it before the update. I do believe that if I was able to roll back to IOS 15.2.1 that my 8+ would be working the finest yet again. And by the way, I've just received notification on my iPhone 8 to update to IOS 15.4.1 ummm I don't think I will bother.

No.