Just a point over train lengths.
Most stations are built and signalled around the concept of 12 x 20 metre coaches. (That has applied south of the Thames since the 1930's.)
With terminal stations this really restricts the profitability of charters. Many stations do not have the space for special working instructions allowing for the loco beyond the platform starting signal, it would occupy track circuits and lock the job up.
Victoria, Paddington and a lesser extent Euston with longer platforms are therefore more attractive to charter train promoters.
The formation in a 12 car platform would be loco, PO Brake, 9 coaches, top and tail DL. Now allow that 2 of the revenue coaches are kitchen cars for the dining element of the train. Say 5 dining coaches (210 pax) and 2 "enthusiast" (128 pax) and it's obvious why charter fares are climbing steeply.
However the promoter does their sums, "the profit is in the last coach". If you loose the flexibility of an extra vehicle or two, there goes any profit.