for what route(s)? There are no routes currently in TSW that are suitable for the IETs.. The closest candidate is GWE but that is set in 2015, 4 years before IETs replaced HSTs on the GWR network
Class 800 is also good from St. Pancras to Faversham, know this is wrong route for it, but there is possibility to change from electric to diesel and vice versa
That's a job for scenario planner when the 80× comes - when a route is appropriate for it. Not something you get a DLC out of.
Just so you know mate Lumo have 803s not 800s and EMR will have 810s I believe but definitely not 800s. It would however be great to see an 80x in game at some point as they are forming the backbone of intercity services in the country these days.