Why, in this country, do we seem to be incapable of doing any transport project properly and at the time that it is needed ?? This applies equally to road, rail & air. This idea was first mooted in the late 90s, along with a flyover/under for the ring road where it crosses the M32. If it had been done then it would have only cost £42.50 !
£428 million !! A better idea, buy a purpose built fleet of 5 car (which can be extended to 6 in 10/20 yrs time) dmus with full through corridor, 4 abreast comfortable seating, adequate leg room, a good scattering of fixed tables, fold down tables on the airline seating, working air conditioning and a mini buffet (

) for Cardiff - Portsmouth then save the rest for a Junc 18A scheme when the idea is re-floated in 2038.
Nah ! DfT will cascade the 5 car IETs to that route in 2064 and save billions so they can electrify St Erth to St Ives. (HS7)