2020 - Final Thoughts
Electrification
Obviously there's a lot, lot more of it in this TL. And it's the older standard of 1.5kV DC largely (bar Pullman). 1.5kV DC was the original proposal for widespread electrification, but it does have several disadvantages over the more modern 25kV AC; a thicker overhead wire is needed for the higher current, and you need feeder stations more frequently. On the flip side, it's a very simple system and works well - it has higher power (and overhead instead of third rail) over the OTL south eastern third rail system, and that has gently expanded over the decades. The 1.5kV DC system with makes it well suited to urban/city network electrification, which we've done often here - most city areas are electrified, and some of the main lines. Roughly speaking, there's been one city network and one long main line electrified every 10 or so years, with an interest in creating wider networks, eliminating short stubs of non electrification (so that fully electric trains instead of hybrids can be used, and the hybrids can then be sent elsewhere).
I see electrification being much cheaper than OTL; OTL is about £1m to £2m per single track kilometre (stk), depending on complexity. Most projects in Europe run far below that, as they retain institutional knowledge in the supporting works (erecting gantry in stations, etc etc etc); overhead electrification only projects in Germany and Switzerland are recorded at an average of approx £400k per stk - less than half the cost in OTL of GB. Part of this is due to the wider loading gauge, so granted it's a little easier, but also the financials are easier to sort rather than the complex contractual refunds Network Rail pays to close railways (and especially if slight delays!), and contracts are awarded far in advance of work starting, so works can be queued up and teams move from one project to the next, retaining experience and skills. In light of this, I think in this TL, the rolling works of BR to electrify will make the cost of electrification far cheaper (maybe by todays standards around £0.75m-£1m per stk), which allows a smaller pot of recurring taxmoney to pay for continuous electrification works.
Trains
Compared to OTL, there's more electric traction as discussed. This means more electric multiple units, but also the wider spread means that hybrid multiple units are more popular as they run under electric wires for a greater proportion of their journey, so investing in dual/hybrid traciton is worth while. I think you'll see "modern" electric multiple units, whilst hybrid trains will be older units, like the Sprinters we still have around, which are moved around and continously used - no point in buying new ones as electric wires spread. There's much less variety in train types - urban, regional, Intercity - rather than the cocophony of train types we have in OTL. The fewer types means longer production runs, meaning greater supply chain reliability, more compatible systems (from memory I think in OTL there are 5 or 6 different, and non-compatible, multiple working signalling/coupling types), and easier serviceability.
Route network
Wider than OTL; but bear in mind a large number of cuts in OTL came *before* Beeching, which have roughly speaking stayed in place in ITTL. They closed for good reason; small, unused, rural, money-sinks. Other routes stayed open initially, but there was still a "slow burn" of routes, so more stayed open, but some continued to close - although some which closed were either retained for re-opening, or passed to local Govt control for local transit. Obviously the eye-catchers are the Great Central staying open and becoming a Pullman route, the Waverley Route in Scotland (and Aberdeen network). But lots of smaller routes too. There have also been losers; the East Coast never got it 1980s upgrade. Local transit; the urban networks taking away BR's de facto peak time railway in regional cities is the big winner, with many local transport authorities taking over networks. Manchester, Liverpool, Newcastle, etc etc - they all have decent networks now, and BR gets to walk away from complex peak time networks which are far less used during off-peak. BR more concentrating on regional and long-distance service.
Costs
I guess this is a big one. I'd like to think ticket price would be cheaper; I definitely feel a decent portion of OTL ticket cost goes on "artificial arrangements" - contractual commitments, services (ie. HR for each company), train leasing at inflated ROSCO prices, etc etc. And the wider electrification will reduce fuel costs which are a significant part too. However, BR is a public body, and they never do well at union disputes or business efficiency it has to be said. So maybe a bit lower, but not that much. I think passenger numbers will have risen massively in the 2010s, as per OTL, and significantly surpassed OTL; instead of 1.75bn passengers in 2019, maybe over 2bn. More routes, larger Pullman trains, slightly cheaper costs, and much better service in the populated Transpennine region. I'm a firm believer that the post-privatisation OTL rise in passenger numbers was driven by the economy (and road congestion/congestion charging) rather than anything specific the train operators did, so that would be reflected on BR tool.
Railfreight
I think Speedlink (ie. freight wagons) was a dying network, but may have continued if it could have been converted to container boxes instead for rail operations, and possibly marshalling yards (electric wire-free) with cranes moving boxes between trains quickly and efficiently. Who knows. I think container trains from docks to inland yards, and via the Channel Tunnel would have been greater, especially with Pullman taking many of the high speed trains out of the way, and a greater route network to avoid busy areas - and wider electrification for cheaper operation.
Privatisation
I think something was almost guaranteed in the political climate of the 1990s. The small route operators are perhaps the best way of doing this; they create networks which are financially viable, long lease times encourage the private operator to invest in the network with a real chance of getting a reward for it, and also galvanise opposition to wider privatisation of BR!
And there's a brain dump of remaining thoughts I've churned out for 15 mins, just to bring this to a final close after over 2 years! Maybe an appendix or two if I have time and the motivation one day!