Based on existing London flows to/from Mansfield (and Worksop), I would say a definite NO for a direct London service:-
LONDON FLOWS (total both ways) 2023/24
Mansfield 14.620
Mansfield Woodhouse 3,404
Worksop 7,544
Compare these with 2023/24 London flows via nearest mainline railhead alternatives for both towns
Nottingham 914,466
Newark Northgate 538,592
Chesterfield 219.478
Retford 222,644
It would appear (to me) that a disproportionate number of London passengers in both towns are already using railheads that offer a frequent and direct service, and that no rail operator would want to introduce a direct London service based on those figures.
Even more remarkable when you consider the populations of Mansfield compared to Newark (100k v 30k) and Worksop compared to Retford (60k compared to 20k). The same would apply comparing Nottingham v Newark figures. As many have already made the point numerous times on this site, it is not all about population.
Also, just how much would those Mansfield (and Worksop) flows have to increase by, and to what frequency, to justify direct services to London?