In the UK at least, as long as you meet the requirements for road safety (which are remarkably little, for custom cars), you can register custom vehicles on the road. We have a wonderful history of insane people building cars in their sheds: https://www.youtube.com/watch?v=0DGtf7asP8I
Yes, you get it approved at the 'RDW'. Checking the plates (N-784-BX) at https://ovi.rdw.nl/ gives you a bit more info. Like, the list price of €4.235.000 :-)
