How about something for the existing network. Ruling speed upgrades for all lines possible, to try to make sure that the car is never faster.
Primary main lines
At the next major relay, upgrade all sections of primary main line (ECML, MML, GWML, Reading-Penzance and any non-tilt WCML) to 125mph with TASS. Also consider structure upgrades to enable the majority of freight trains to run at 75mph. Where there are slow lines, passenger trains should run at 100, freight at 75. Vehicles conforming to coaching stock requirements should be able to run at up to 125 even if non-tilt where line curvature permits.
Secondary main lines/Major commuter routes/Regional express routes
These are lines such as the GEML, WAML, Kent Coast Lines, Brighton Line, SWML, Golden Valley, Cotswold, Holyhead, Chiltern, Trans-Pennine, Scottish 'Express' routes and so on. Increase passenger ruling speed to 100mph, with a freight ruling speed of 60 (currently, many of these lines have an MU or SP derogation) wherever possible at the next major relay. Dual all remaining single-line sections.
Regional routes/Commuter lines
These are the majority of passenger routes throughout the country. Specific examples include the S&C, Cambridge Branch, Norwich-Ely-Peterborough-Syston and so on. At the next major relay, increase MU or SP derogations to 100, with 60-75 for other types of traffic. Lines with extensive cross-country traffic operated by Class 170s to be considered individually for possible upgrade to 'Secondary main line' status, enabling Voyager operation.
Diesel lines in the former NSE area
To be electrified ASAP, enabling a total withdrawl of DMUs from this area and a cascade to other locations.
Regional Suburban Lines
Specific examples being withing SYPTE, WYPTE, GMPTE, Valley Lines, etc. Aim for 'harmonisation' with the former NSE area. The principle being that these cities should be considered on an equal footing with London for provision of suburban rail services.
Rural branch lines over 50 miles
This category was created to deal with anomalies such as the long routes in Scotland and Wales, plus the Heart of Wessex and so on. If the line is RETB-fitted, follow the course for lines under 50 miles. If non-RETB fitted or double-track, consider an upgrade to 'Regional routes' status.
Rural branch lines under 50 miles
At the next major relay, increase SP derogations to 75 mph, allowing current stock to run as near as possible to its maximum speed. Lines with extensive freight and excursion traffic should see general passenger speeds increased to 60 and freight to at least 45, to avoid holding up faster passenger trains. Consider an equivalent of RETB fitment to ease maintenance costs.