I'll just push back a wee bit
on the cost-effectiveness issue.
For loudspeakers, one may use fairly expensive drivers (and -- ahem
-- XO components and wire, too, if one wishes) without paying the fully-built loudspeaker premium (markup) for the drivers. That said, of course the commercial loudspeaker manufacturers negotiate volume discounts, and
their gross margins also allow them to be very picky about selecting (only) drivers that meet fairly stringent QC criteria (if they so choose, of course), so it's not quite all unicorns and rainbows -- but chances are you'll pay $5k or so-equivalent for a commercial manufacture to use a driver you can buy "off the shelf" for $500. Give or take.
Bean counters do operate at all levels in the industry -- yes, probably even at the Wilson and Magico strata.
So -- I dont' know of good UK equivalents, but in the US, e.g., we have Madisound https://www.madisoundspeakerstore.com/index.php
, which is a pretty good one-stop-shop for DIY loudspeakers and kits. In a more "beer and pretzels" economic niche, there's also www.partsexpress.com
of course. They've expanded into flatpack kits in a big way in recent years. My biggest beef with PE is that virtually all of their designs are low-sensitivity.