You are maybe thinking about  it in slightly the wrong way - there is a limited amount of space in the regs for tables, and there are not experimental results available for all cables in all configurations - actually to do well it is quite hard to test a cable - slitting and resealing it it to add thermocouples and running many samples in a controlled environment at different currents.

It does not mean that you can only use cables in the tabulated types in the tabulated installation and routing methods, just that there is only guidance for a representative sub-set, that is the ones that have been repeatedly tested and agreed upon.  Generally cable on trays is multicore in the smaller sizes but of course it does not have to be.

Of course when you do your own design you will have to decide how close to the edge you want to sail and if one installation method is close enough to another for your purposes. So if perhaps  you wanted 16mm2  on a tray, you could probably use the figures for 16mm method C for example and add on a similar percentage as  the equivalent ratio of methods C to F for 25mm2.

Mike.

IS10101:2020 (the Irish Regs) is as per 7671 in this regard but provides table 52C as an example of simplification of the the requirements of Section 523, in particular regulation 523.2, which is much the same in both documents. 
For reference method F single phase, 52C reads; 23, 31, 42, 54, 75, 100A for 1.5, 2.5, 4, 6, 10 and 16mm2 respectively and 19.5, 27, 36, 46, 63 and 85A for 3-phase.
There is no indication as to why 1.5 to 16mm2 is omitted from the comparable table to 4D1A in the Irish Regs but I reckon Mike is right, he usually is!

IS10101:2020 (the Irish Regs) is as per 7671 in this regard but provides table 52C as an example of simplification of the the requirements of Section 523, in particular regulation 523.2, which is much the same in both documents. 
For reference method F single phase, 52C reads; 23, 31, 42, 54, 75, 100A for 1.5, 2.5, 4, 6, 10 and 16mm2 respectively and 19.5, 27, 36, 46, 63 and 85A for 3-phase.
There is no indication as to why 1.5 to 16mm2 is omitted from the comparable table to 4D1A in the Irish Regs but I reckon Mike is right, he usually is!