Pre-1923, I have content for:
NER - Paulz Trainz has pretty much all the NER engines you could ever need
NBR - v. high quality, edh6
HR - ditto
LNWR - ditto
GWR - skipper1945
LSWR - ditto
LBSCR - jayholland has done loads of LBSCR stuff
Great Central - camscott has done a few types
I think for pre-grouping fans, the only ones missing are the GER (only the J69 and the Claud), the Midland (only the Compound, a 0-4-4 tank and a flatiron), the Furness, the Glasgow & South Western (a truly unique little line actually), the Cambrian, and the Hull & Barnsley.
There's nothing significant missing for the grouping period (apart from very esoteric things which are usually my favourites!) with the LNER, GWR, LMS, and the Southern represented well. I can't think of one type that these companies introduced in significant numbers that doesn't have a decent model in Trainz apart from the Saint, Star and County classes of the GWR.
BR period, again there's plenty. I think the Standard 4MT tank deserves a new model but all of the classes are available from Paulz Trainz and the DLS. All the diesels apart from the prototypes and the Class 15s can be found on the DLS, even the D600 warships and the Baby Deltics which were famously unsuccessful. Warships, Westerns and Hymeks could all do with new models at some point, I don't doubt we'll see them though.
The only gap is AC electrics, DMUs and EMUs. For a phenomenally successful unit, the 101 on the DLS is ancient and 105s aside that's it. EMUs are doing a bit better but suburban routes around London, Glasgow etc. can't be done as with one or two exceptions the stock isn't available. The first generation of AC electrics has not yet been done (at least not in original condition) nor have the EM2s/77s.
The only period you'd struggle with is ironically the one we're living in. Modern stock is available but especially in the area of passenger MUs we're lacking Electrostars, 170s, numerous 158 liveries, 91/MK4s (yes there is one but it doesn't work in TANE or TS12), 70, IEP etc. So I can only conclude the OP wants modern era stock as that's the only area where Trainz has content gaps, UK wise (not that I'm complaining as the modern scene isn't that interesting to me).