Network Rail seem to think London trains need to use platform 0 in order to make sufficient capacity for the expected increase in traffic, including an extra two trains per day to London in the morning peak and back in the evening peak: They suggest that platform 0 be the main platform for eastbound services, with platform 2 being the main platform for westbound services and platform 1 being available for services in both directions.
So it appears that the problem is Network Rail have decided that platforms 3 and 4 will be dedicated to releif line services (with the latter also being shared by the ValleyLines) and thus unavailable for use by London trains. Elsewhere in the document, they suggest that Ebbw Vale services (2tph), 1tph between Cardiff and Bristol and a Cardiff-Cheltenham service would use the relief lines after upgrading the linespeeds on them to match the mains. That doesn't make sense to me, because the Ebbw Vale services would then have to cross the mains at Ebbw Junction. Wouldn't it make more sense to put the Cardiff-Nottingham and Cardiff-Portsmouth services on the relief lines instead? Platform 0 could then be used for trains to Ebbw Vale and the marches line, leaving platform 1 free for London services.
The problem with doing that, I guess, is that the proposed new station(s) between Cardiff and Newport would require platforms on all four lines, not just the reliefs, so that the Ebbw branch services could call. Also, moving Bristol services to the relief lines would result in down trains crossing the main lines at Severn Tunnel Junction to reach the down relief (up trains have a flyover; could that be made bi-directional or is it too busy?).