The Celts did in fact inhabit most of Europe in pre-Roman times, or at least very big swathes of it. For example there are some stone-made Celtic-crosses that are popular in what is today North-Western Romania (Maramures county), there are different articles about them (in Romanian) which also include some photos [1]. Now, I didn't know if that same type of cross was also present in Scotland but a quick search directed me to this article [2], which tells me that they used to be quite popular in early Christian times. The county of Maramures is pretty close to what used to be Scythia.
I've found another blog-post with more photos about the Celtic crosses from Maramures [3], and in the comments in there someone also linked to similar crosses being found in Northern Greece/Macedonia, close to Thessaloniki [4]. Very interesting stuff.
[1] https://www.bzi.ro/morosenii-au-origini-celtice-artefactul-c...
[2] https://www.churchofscotland.org.uk/news-and-events/news/201...
[3] https://www.atlasobscura.com/places/abandoned-bogomils-grave...
[4] https://www.atlasobscura.com/places/abandoned-bogomils-grave...