The Sussex Route sees a fair amount of closures of the likes of Three Bridges-Brighton, South Croydon Jn-Uckfield, Horsham-Barnham and so on. However, as has been correctly pointed out above, often the worksites and possessions are in much smaller areas, distributed as efficiently as possible so that they are grouped together by line-of-route. Often there can be a portion of the route accessible (particularly at one end) for shunting manoeuvres or suchlike, but due to the lack of infrastructure which is able to be used for an efficient passenger service, closing the route to passengers can be a more effective option to use.
Sometimes the lack of ground frames and crossovers in some areas (generally as a result of over-rationalisation) does not help matters, but even if they are there, often you would be terminating trains at unstaffed or part-time staffed stations with poor passenger access, poor bus access and so on. Running buses between major stations is often the most sensible way of doing it, even if it extends journey times. I'm no replacement bus fan myself, but I know why some places are chosen above others (or should be)!
