I think there's an element of "be careful what you wish for"... demanding that planning processes are sped up significantly may only create an environment where rail schemes are easier to reject out of hand (rather than keep making positive noises about them without actually progressing them at all)
I'd also sound caution regarding Scottish comparisons - plenty of places in Scotland haven't had their old railways reinstated yet (or new towns/ Airports that lack a proper station)
It took almost forty years between the Borders line closing and the 2006 Waverley Route Act passing at Holyrood, and then actually opening in 2015 - and this was just an unnelectrified partly single tracked line covering only half of the Waverley route which doesn't even serve the biggest place in the Borders, on what was probably the most high profile "Beeching" closure in the UK (with all of the "political" aspects of the Borders region having been left behind with lots of knitwear jobs being moved overseas, the traditional Liberal territory giving the region a strong voice... even with all of the arguments about an entire region not having a single station, it took almost forty years of campaigning to get it approved and then almost another decade for it to actually open).
Whilst I appreciate that anyone focussed on their pet local project in England will find that things don't go as fast as they would like, the Borders experience doesn't suggest that things move at lightening pace in Scotland either.
The short freight branch from Thornton to Methil is only now getting approved, but I'm sure the people of Peterhead/ Fraserburgh/ St Andrews/ Grangemouth etc would take rise at the "this would have been re-opened by now, if it were in Scotland" reaction that people trot out on threads like this one about Portishead. There's a long list of projects that haven't been done north of the border ... the grass is always greener
That said, Portishead makes sense in theory - it's a shortish line from a decent sized town to the nearest big city, there's a frequent enough commercial bus service to suggest that there's a market for public transport, it stands on its own two feet (i.e. there's no bluster about it being "useful" for diversions on a couple of weekends a year, it's not some romantic route rambling through rural villages, it's a simple enough plan that belongs in the same bracket as Ebbw Vale to Cardiff or Ashington to Newcastle) - but I don't know the local conditions so can't comment on the best location in the town for a station etc
However I'd rather we focussed on getting the knitting up at Temple Meads before any "new" projects are taken on in the area
Whilst the emissions from the individual train may be worse than people using their cars it's likely not that straightforward. Especially given that to be the greenest when driving requires an electric vehicle, which is still very much in the minority.
First off whilst it may open with DMU's in use that's not going to be the long term and so over time it's going to reduce emissions beyond what would otherwise be the case
Can't have it both ways though - you're pointing out that only a minority of cars are currently electric but also that the railway might eventually be electrified one day (by which time presumably there'll be a few more EVs on the road)... given how the backlog of electrification has got, I wouldn't imagine any wires to Portishead any time soon