It is quicker and less disruptive to build new lines than upgrading/widening old ones, as HS1 showed. Widening bridges is particularly troublesome, to modern building standards and 25kV clearances you usually have to build a new one, which disrupts existing road traffic as well as rail.
The positive however is that widening does not incur the NIMBY resistance and long planning delays. New lines which speed between big population centres without stopping are of no use to local people who are affected, hence the paradox that new roads face less opposition than high speed lines, although they are more environmentally disruptive. High speed lines (300km/h or better) cannot even curve as much as motorways, so any route through crowded England is going to destroy housing, or else involve very expensive tunnelling + pressurised trains like HS1.
The other point is the economics of really high speed is that a 200mph train requires nearly three times the energy of a 125mph one, whereas it is only 25% more for 140mph. Energy ain't going to come cheap again, and we have a looming capacity problem with electricity generation.
Having gone through the pain of rebuilding the WCML, with new signalling new wiring and power supplies, new track, there really is no excuse for unreliability. Anyway what do you do with those Pendolinos, which have another twenty years life at least? Stoke/Macclesfield/Stockport could be served from a loop from HS2, but Milton Keynes and Coventry could not. And you still could not get from London to Glasgow in three hours, because no way would HS2 be justified beyond Lancashire.
Oh, and the country is bust.
The problem is freight capacity. Doubling the Chiltern line was supposed to solve that, but given all the new passenger trains that has created a new bottleneck, which is Moor Street - Aynho junction.
The real capacity crunch is south of Rugby. My suggestions:
NO to HS2.
YES to overlaying ERTMS on the signalling all the way from Watford to Carnforth, so Pendos can do 140mph nearly all the way
YES to reopening the GCML south of Rugby as a freight line. It's intact and hardly any building on it. Only problem is you would probably need a tunnel under Rugby, as it crosses the WCML at right angles in the middle of town. It can connect to the Chiltern line at Ashenden junction. There is little opposition to reopening old lines.
You could electrify it at low incremental cost, but wonder if there is the demand, as 66s run freight apart from Freightliner.