Whilst signalling improvements are probably needed for through trains across the whole line to run at a higher frequency of service than 3tph, perhaps around 4tph in peaks, to be reliably achieved (bearing in mind the "bitty" nature of the control of NDL trains at present, and the significant variation in signal section lengths and capacity of the signalling system) I think adding that into the current debate does increase its complexity massively. I have a feeling that complexities like the interfaces with other routes and the much longer term potential for cross-country freight would mean that colour-light signalling upgrades, level crossing upgrades or removal, and more uniform signal section lengths would be of greater benefit than ERTMS in and of itself - at least over the next decade or two.
Platform lengths would, I think, be a very achievable goal once disused or near-disused infrastructure such as foot crossings are dealt with. There will naturally be some areas where these are harder, but the overall picture is that several of the major stations could already accommodate at least one more coach than is typically used (maximum formation is usually 3 coaches, though on very, very rare occasions I have noted 4 coaches in use, with local instructions for door locking at any relevant short platforms; the last time, this was to accommodate a major sports event somewhere) - and in the case of stations like Gatwick Airport, Redhill, Guildford and Reading, significantly more coaches than that. Additionally, any electrification bringing newer stock may also give sensible options for SDO, which could help if there are any stations where lengthening is hopeless. We know that NR et al are prepared to majorly lengthen platforms on even very rural routes in the area in order to allow better end-to-end capacity overall; the Uckfield line is a case in point, although of course the trains travel to London Mondays-Saturdays. A big concern of mine, though, is that bus replacements and service alterations for platform lengthening on the NDL would be much more disruptive for occasional travellers, airport users and the economy in general.