The likely ones on that stretch of line are:
 
Class 221 - Payware from Railwaves
Class 90 - On DLS - various authors
Class 321/350 - 321s are available under username nexusdj, the Class 350s are not yet available.
Class 377 - Payware from Railwaves
Class 390 - look under nexusdj for the latest version
 
Freight
 
Class 86 - should be on DLS
Class 66 - on DLS under various authors
 
Class 325 Royal Mail - look under nexusdj on the DLS.
 
If you come to do the Bletchley-Bedford line, you will also need some class 150s and possibly some class 153s (both are on the DLS)
 
This is not an exhaustive list, and there may be more.
 
Shane
 
EDIT: CamJHock, please remove your email address from your first post. If people need your email address, they will PM (private message) you - also, this part of the forum is public (and does not require login to view, so spammers can harvest your email address and send you a lot of spam)