I posted yesterday that the fastest London-Grantham non-stop was around 1hr 6min. Grantham to Nottingham is 22 miles, with severe restrictions on the approaches to Grantham and between Radcliffe and Nottingham, so I have assumed 60mph average speed. Thus the journey time with the one intermediate stop would be about 1hr 30min.
The hourly off-peak service via Leicester is about 1hr 40min with HST timings and three stops, but under a similarly optimistic assumption of one stop and Meridian timings would be down to 1hr 31min as achieved by the 0755 from Nottingham. The benefit of the Market Harborough remodelling would match the Kings Cross time. The fast Nottingham service also serves Leicester so would have to continue to run for at least two thirds of its journey even if Nottingham was served only from Kings Cross, and along with the busyness of the ECML that means there is no reason to run the franchised Nottingham trains via the ECML. No open access operator would be allowed to run this service because they wouldn't cater for any significant new passenger flows so would be classifed as abstractive.