There's not that many around for Trainz, but have a look for class 313 on the DLS by zxr400. There are a few more floating around 3rd party sites that someone will direct you to in a minute... (e.g.
http://www.skodatrainz.co.uk/6.html)
Most UK 24kV EMUs have a 3xx number, with the exception of some dual supply units that are mostly 4xx.
Paul