The problem is that the 2018 555X / 560X models came out last July, less than six months ago. I can imagine current 2018 MBP owners being upset over something better available at the same price less than six months after their MBP was released.Wow... a 10% increase for £315 more...
These should be direct replacements not £315 upgrades. These MacBook pros are getting so expensive its ridiculous that even a mid range option is in the £4000 region. Then add another £399 for AppleCare.
Plus once you have the CPU and these GPUs pinned the likely hood is they will perform far far less than their potential with the poor cooling so whats the point.
Making Vega GPUs standard is something Apple should do with the 2019 models.