Don't 458s do the Ascot - Guildford route?
Well of course they do now, it's a mix of 458 and 450 on weekdays at least, but they won't once they are converted to 458/5s. As mentioned in a few threads, there are to be 36 458/5s, but once they've taken over routes operated by the 29 450/5s there will be some left to internally cascade other 450s and/or 455s.
There are two relevant press releases that list affected routes and services, but taken together the output from the 108 additional vehicles, (60 ex 460 and 48 x 456s), is felt mostly at mainline destinations, they are strengthening services as far as Salisbury, Exeter, Portsmouth, Southampton, Alton, Haslemere.
In the main this is being done by re-diagramming the 444 and 450 fleets to lengthen existing trains, eg 8.450 replacing a 444, or 10.444 vice a single 444, or 12.450 replacing an 8.450 etc... As someone has already posted there's almost certainly an 8.450 set releasing a pair of 159s to the Salisbury route.
Whatever the details, it is certainly no longer the case that all the 456s will extend 455s to 10 car length, as most people originally expected. Indeed it seems the vast majority will
not be running in ten car sets...