If we could invest £10Bn and get £20Bn back for the economy with our money back over time then not a single politician would turn that down, there's actually been a load of support over the past ~5+ years for "levelling up" (AKA investment in the north) but we've not invested much because the returns are abysmal. Meanwhile there's a whole stack of projects with good returns in london, but politically it's really difficult to "give more money to london" so we don't really do them. Bakerloo line extension and crossrail 2 for example.
The Elizabeth line, will over time pay for itself entirely with fares alone, yet has also already added ~£2.50 to the economy for every £1 spent.
This is a absolutely classic example of chicken and egg, network effects and first mover advantage. There's perhaps a case for being absolutely ruthless and just spending an obscene amount of money on Manchester/Leeds/Liverpool. What might it look like?
-No third runway for heathrow, instead BHX gets 2 more runways (or MAN 1 more) and all the funding it needs to become an international hub, including an reduction in APD for 5 years or something. Central London in ~48Mins via HS2 is already committed and being built, frankly that's not that much different to what it practically takes to get from a random heathrow terminal to central.
-HS2 to Manchester AND Leeds (via the branch just above BHX), that would connect BHX to those cities in <30 Mins or so and to London much quicker.
-Trams for Leeds, maybe a metro for Manchester.
-Liverpool>Manchester>Leeds express line (which seems committed).
-Enterprise zone covering all underdeveloped commercial areas in Leeds, Liverpool, Manchester and Birmingham. Give rates, or tax relief on businesses investing, or employing in the zone, planning exemptions, easier permitting.
Anything short of an absolutely insane bet like this, just isn't going to work. Chucking 1Bn to Leeds for them to buy some trams just isn't going to have a transformative effect, it might actually be a total waste because, in short, network effects.
The North of England frequently runs a combined fiscal deficit in the tens of billions, with all of that subsidy coming, effectively, from London and the SE.