Not sure I'd call the period where 91s and HSTs ran all that modern, unless you mean just before the introduction of the IEP.
For the 1990s you can get classes 20, 31, 37, 47, 56 and 58 in N gauge, there are also DMUs 101, 150, 156, 158 and you've got the class 320/1 coming from Revolution albeit wrong livery for the time as it needs a Metro livery.
For post privatisation you do have similar options to choose from too less anything scrapped but we are lacking a TPE 185 and it does get harder if you want to model surburban and outer suburban services from Kings Cross with only the 313 on the horizon.