Hi Stuart,
I'm also in Ayrshire (it's a small world!) so i can certainly advise on this one.
All of the classic B&O equipment i have owned has come from either eBay or Gumtree. Facebook marketplace is a thing now but i don't have an account for that. The best thing for you to do would be to set up an automatic notification if there is new listing matching "B&O", "Bang Olufsen" (or similar) within a 50mile radius of your location.
As Ayrshire has quite a few afluent areas (where people might have bought these systems new) but is otherwise a bit remote from the rest of the UK there are not so many people in competition with you who are looking to buy - look for sellers on ebay who advertise as local collection only.
The other good thing is that you will be unlikely to ever lose money in buying pre-owned B&O equipment as there is a huge number of collectors out there and most of the products work using the same connectors and remote controls so if you can't get a 4500 component you might settle for one from the 3500 or 3300 range and then swap it out at a later date as i have done.
Products i've bought in our local area include Beovision 6 (in Ayr), Beomgram CD4500 and Beocord 4500 (both Largs), Beosystem 2400 with original cabinet (Troon), Beosystem 5500 with Penta speakers (South Glasgow) and Beocenter 8500 (Hamilton) so there are definitely plenty of items out there.
I've found that B&O Merchant City in Glasgow have been very helpful too and regularly sell preowned equipment on their website (although this tends to be newer than the vintage era)
I actually have some old equipment which i no longer use - mostly beolink ML and MCL cabling, speaker wire etc which you are welcome to have if you find a system to go with them.
happy hunting!
Scott.