I disagree that was a hard rule at least in BR WR days.
Indeed I believe it is a myth at least in more modern times, or something that has been misunderstood and or misquoted over time, although it might have been the case way way long ago.
If it as you say it is, then there must have been quite a lot of WR express passenger trains over Dainton and other South Devon banks that had Moguls or Prairies etc as train engines and Kings and Castles as assisting engines - for many of the BR WR are pictures you can find on the web have the smaller engine in front of the bigger engine which you say train engine in front of assisting engine. And I'm pretty sure they did not faff at Newton Abbot doing any more than detaching (Up) or attaching (Down) a pilot.
Likewise every image I can find of steam assisting freight through Severn Tunnel in BR WR where there are two engines on the front are of a Prairie piloting a 28xx/38xx or Hall etc - the Prairie surely is not the train engine which is what you imply, especially when STJ Prairies were based there as the normal assisting power, the 28xx/38xx is the train engine.
The rules were set out in the WR Regional Appendix:-
Assisting or double heading may take place only ... ... i.e. on main lines and other routes detailed in the Sectional Appendices
Passenger trains
The engine required to assist or double head may be coupled in front of the train engine, providing it is in the same group or superior as below.I f this is not the case, the second engine must, with certain exceptions, set out in sub-sections (ii) and (iii) below, be placed inside the train engine.
In broad terms from superior downwards a) 4-6-0, 4-6-2 b) 4-4-0 2-10-0, 2-8-0 & T,2-6-0 with bogie/pony truck leading: 2-8-2T, 2-6-4T, 2-6-2T either direction, 0-6-2T with bunker leading and most inferior c) 0-6-2T chimney leading, 0-6-0, 0-6-0T
(ii) Tender engines of the 2-6-0 type with driving wheels not less than 5' 8" (my note - 4300 were that diameter) may assist in front of any engine except Kings over a variety of lines but including Totnes-Newton Abbot, N.A to Brent and Plymouth-Hemerden.
(ii) Tank engines of the 2-6-2T type with driving wheels not less than 5' 8" ( so that limits to big prairies except 8100) may - a list then follows, same as the 2-6-0 plus additions.
Freight trains - too much to type! but broadly similar.
Wasn’t the rule only hard-and-fast when the pilot engine didn’t have a pony-truck leading?
Having the pony truck leading (eg 0-6-2Ts had to be bunker first) was a parameter but if they didn't - i.e. group c) - , then the only loco that they could pilot was also one in group c).
So there were situations where the train loco had to be moved 'inside'.
Edited to add the keys (ii) and (iii) missed that in the copy typing