Reading the Enhancements Delivery Programme (Thanks
@jimm for posting the link), it looks like the Bristol wiring and the Oxford wiring is set to occur in CP6; the resignalling schemes in the next half a year will pave the way for the improvements in those areas. The Thames Valley Branches (not Greenford), along with Southcote Junction to Basingstoke have also been deferred to then, according to the EDP slideshow - I'm eagerly awaiting the March '18 update.
Cardiff-Swansea doesn't seem to be so lucky however.
I have the memory of a sieve when it comes to why the section west of Cardiff got "Graylinged"; I assume cost overruns combined with a lack of concrete answers on what the Welsh Gov't wanted in the Valleys had something to do with it. (If anyone knows more about it, do post it down - I'm sure I'm not the only one who needs a reminder!)