In defense of Marcus his statements: yes, with all the connections to AMS from airports around the UK it can be much more convenient to fly via AMS.
We can all find examples where a flight under specific terms like time of year, market conditions, available seats, competition, etc. is cheaper via a certain airport. It’s called competition, I guess.
Unfortunately for people in the London area LHR has very high charges and, probably, due to that the cost of a ticket is almost always considerably higher than flying via AMS. Flying from the North I always have the option of going through LHR or AMS and based on the quotations from our travel agent and what I can find myself on the Internet: flying via AMS is almost all the time (much) less expensive.
I checked the economy fares for a trip I plan to make later this month to LAX for a week. (I don’t book my own trips, we use a travel agent.)
25 june and return on 3 july
BA LBA-LHR-LAX rtn £1069
KL LBA-AMS-LAX rtn £ 847
The prices differ from day to day, but seem to be changing on an almost daily basis as well. (That is why we use a travel agent. They get even better prices and conditions.)
I also noticed that KLM seems to be charging more to people travelling direct from Amsterdam than for people travelling on feeder flight, like I do. From your little list it looks like BA is doing the same!
Maybe this is a common practice for airlines, but I can’t think of valid reasons behind it.