Derby and Nottingham are very much 'M', as far as headcodes are concerned (and despite the ever-changing boundaries of the Network Rail 'routes'!), as evidenced by the various 6Mxx freights to Mountsorrel and Toton. WTTs certainly used to make reference to "the former Anglia region", for example). Even if it were considered as a service entirely within one region, I'd still suggest that a 1Sxx headcode would be inappropriate - with the exception of class 2s (where, as above, special 'rules' seem to apply owing to the smaller number of long-distance examples), E, L, M, O, S and V should be reserved for interregional services, apart from a few cases where they're used (1Sxx on the Southern and 1Lxx on the Western) where they're not likely to encounter an interregional train with the same letter!
. It does seem as though a pretty robust system is gradually being diluted by some planners who neither fully understand it nor have to use it to quickly and reliably identify what's going where! There's little scope for confusion with an 'internal' class 2 running as 2Sxx, but the same can't be said for class 1s (or, indeed, freights - which seem much better at following the conventions).
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That sounds like a 'tea with no biscuits' meeting in line for those two then 
