also I'd expect that flights from Manchester and Birmingham to (say) the USA would be cheaper - but when I look the savings seem to be minimal over LHR
Why on earth would you think that ?
The basket of fares from Manchester to the US is and always has been higher than those from London.
In olden days scheduled price were based on ex London plus miles basis to everywhere.
Today the matrix differs and yet the basket remains.
As constantly pointed out there is way to much capacity in the back of the bus out of London and this is dumped at a level simply unmatchable even with an added on shuttle element !
The market dynamic of Hub and Spoke necessitate competitive fares over the hub
28 daily flights between just one city pair specifically corrupts the whole market place.
No your premise is very wrong.