I'm not sure if there are fewer horse owners in the UK by ratio of the population but as a very small country we are crammed together more and owning a property with enough land to even keep a horse is beyond the pockets of most owners so less amateurs breed horse. That doesn't mean there aren't plenty of people who do breed bad - quantity above quality but as sales have dropped so have numbers. A lot of these types were/are ponies that cost little to produce and often end up straight for the meat market so never get into the breeding chain.
One of the biggest problems are the owners that get an outgrown pony or buy a cheap failed racing mare and think that by putting it too a nice stallion they will get a good result.
There is a lot of Irish Draft and welsh cob influence still in UK breeding and they do have a better genetic back conformation which might also explain why there are less faults seen
IMO all stallions and mares should have to be officially approved for breeding and licensed as such.
Back to the horse on the right - there is a term called 'jumpers bump' in the UK (might be universal) that does look like that horses shape. Its generally a result of jumping strain either from over exertion - which is why its often seen in (fox) hunters and TB's that race over fences and horses that are jumped too early and then too often. Its caused by a strain to the ligaments at attachment of the lumbar and sacral vertebrae and though painful at first it does harden into normal scar tissue but may reduce the horses ability to jump though not affect its general use as a riding horse - though in UK show classes where horses have to be stripped of tack & also judged on conformation they wouldn't be any use
This is a link to one web page that explains it better and has a large photo http://www.jwequine.com/pdf/hunters-bump.pdf