Hi, and welcome! Either retrobike or ebay. Beyond that t kind of depends what you want to do - squeeze the maximum possible out of it or find it a good home. I'm not an expert on the retro road bike market but while Principia are quite well regarded and have a certain following, the bikes just don't fetch that much, as you'll see if you search for sold listings on ebay. I've owned one myself which I wish I hadn't sold now, but I think the ones like this that have the garish yellow logos just aren't that attractive! These bikes are in a difficult in-between position in the second-hand market as they don't really have the allure of real retro road bkes (too modern) and as modern bikes they're just superseded by anything carbon and with more recent groupsets (not modern enough).
Splitting the frame and parts might get you a bit more out of the bike overall, but it's not a strong marketplace right now and there's no guarantee at all that you would sell all of the parts individually, and/or it could take a long time till you sell everything. It doesn't help that the components are mostly mid-range and from a not very sought-after era, despite being in good condition.
So I would say sell it as it is and hope to find someone willing to pay a couple of hundred quid.
