Remember that construction finishing is only halfway. You've got to commission and test the systems, then train the staff and test the processes (e.g. evacuations). You've also got to do gauging checks. Once all that's done, you have a trial running period then a preview service anyway.Yes it does seem a fairly unambitious target, looking at construction progress on main civils so far. Can’t be that long before Winslow station build starts.
Given the level of interaction with Oxford and Milton Keynes, especially with Oxford remodelling, the timescales aren't necessarily unrealistically slow.