I still don't understand this. The same all-in flight from LHR costs only GBP 155.20 - LHR-AMS-MSQ, yet the first leg LHR-AMS on its own costs GBP 97.30, which is more than the LCY-AMS one-way flight at GBP 76.12. It follows that LCY-AMS-MSQ should be cheaper than LHR-AMS-MSQ, whereas KLM are actually charging 9 times the price for LCY-AMS-MSQ.
But it doesn't follow.
Back in the old days of regulated fares, it may sometimes have been the case that fares were cumulative, in the manner that you seem to think they should be.
Each airline ticket is a unique item with a different market interested in that particular product. The actual "costs" to the airline may be cumulative in this manner, but airlines will not (always) merely price connecting itineraries in the simplistic manner you expect. Every airline practices revenue management, and will set the prices as the market dictates. Every airline will attempt to earn the maximum price that they can - not just cover their costs. In general, this means that direct routings are offered at a premium over indirect ones - but not always.
The person who wants to fly from LCY to AMS is different to someone who wants to fly from AMS to MSQ, and - apart from the fact that KLM can only offer flights connecting in AMS [not really - they can offer options via CDG and other places, too] - is also different to someone who wants to fly from London to Minsk.
Additionally,
KLM doesn't fly to MSQ at all, so the ticket they are selling is on a partner (Belavia), and their agreement to sell tickets on this carrier may be in part determined by where that passenger is originating. Belavia aren't going to want to find that their own London market is "bleeding" over to nearby hubs where they want to carry that traffic directly themselves on their own metal.
If you really want to fly via Amsterdam, why don't you book your travel on separate tickets,
Perhaps you may have seen on the BBC One news this evening that a one-way train ticket from London Kings Cross to Sheffield direct costs 56% more than buying separate tickets from London Kings X to Doncaster, and Doncaster to Sheffield - and even arrived 10 minutes earlier.
It's so frustrating that there's nobody at KLM who can sort out what must be an error or oversight. They'll lose my business and I'll lose the opportunity to fly from a convenient airport. It's lose-lose.
It's clearly not a route that KLM care about - even more so when they don't actually serve Minsk themselves at all. And looking at the pricing of all airlines, the London market for Minsk wants to depart from LHR or LGW - not from LCY, so this "discerning" market that can't travel west are willing to pay far more cash.
if departing from LCY is of such importance to you - you are paying a premium of at least 100% as compared to leaving from either Heathrow or Gatwick, based on a quick survey of fares from each of the London airports - then I don't see why you are also hung up on KLM's fare, when there are cheaper options available on other carriers from LCY.