On BA all business class tickets are changable for a nominal fee (about 5% of the cost of a return ticket). It's that flexibility that means people pay £6k return for the cheapest business class LON-NY ticket.

Sure, if you're flying on the "cheapest flight on skyscanner" then you don't get the flexibility, but that's why they don't fly 6 A380s rather than 12 smaller planes, they want the £6k tickets to choose BA/AA rather than VS/DL. The £600 return tickets at the back will fly whoever's cheapest.

>> BA all business class tickets are changable for a nominal fee

That's a sweeping claim.

Pick a highly-discounted business class fare LON-NYC-LON for dates well in advance which books into I fare bucket.

I close the ILX1C1S6 fare, out 25 Dec 2019, back 1 Jan 2020. (The base fare is GBP1271 r/t...)

Either do a dummy booking at BA.com, or go to ExpertFlyer[0] and the fare rules. Find the 'PENALTIES' section:

  CANCELLATIONS
   
     ANY TIME
       TICKET IS NON-REFUNDABLE.

[...]

  CHANGES

     ANY TIME
       CHANGES NOT PERMITTED.

[0] www.expertflyer.com

I looked at a selection of business class fares (including fares booking into I buckets) a couple of weeks in advance and all were changeable for £300.

"Almost all" then, especially when you factor in corporate deals

Which route(s)? Were you including a Sat night stay?

At BA.com I've just looked up LON-NYC-LON out 6 Mar back 13 Mar (so just over two weeks from now), and I'm seeing the same complete lack of flex:

Your ticket conditions

Changes to your ticket No changes permitted. No upgrades permitted

Refunds There are no refunds except for any government & airport taxes