Chemical plants aren't active in the 1830-1870 time span, so adding them to the map would not be useful - they would not produce chemicals until after 1900. Munitions factories begin by using 1 iron = 1 ammo; in 1856 they go to 1 iron or 1 steel = 1 ammo.
The 12mph requirement is, as you say, historic. Keeping locos updated to the newest model will usually do the trick, along with some judicious double tracking (Manchester, Birmingham, London, Bristol). I usually double-track the entire length from London to Birmingham, for example, and in extreme cases I've build additional stations (London and London East, for example). You can use the density filter to see which rail lines are heavily used (red and dark red); these are the ones that need help.
Once the Firefly comes available, converting over to that engine will largely solve the speed problem with trains of 6 cars or less. The 'fine' is $50k per month, or $600k for a whole year; it is annoying - by design - but not crippling.
You will need lots of trains... I usually run four or six on the Liverpoole-Manchester route alone, and eight as London-Birmingham expresses, with more on the Birmingham-Coventry-Oxford-Slough-London route. A side venture into Ireland can be very profitable, as there are usually enough varieties of cargo there to justify building industries and rails. I don't know about real-life, but in my games Lurgan always ends up a major industrial city in Ireland.
Coal is king. Get above 50 loads per year as soon as you can. This will trigger an event to permanently increase coal production in a large way, plus it is coal that determines how productive your factories are. With 50 loads or more per year, your raw materials will multiply and your factory output increase... in my last game I had 2 expanded steel mills each making 40 or more cars of steel per year.
Industries are important. From the start I usually connect Liverpoole to Manchester, buy a textile mill there and set about hauling cotton/cloth on dedicated trains, in addition to the 4 trains for regular cargo. Rapid expansion to Wigan-Preston-Lancaster, and south to Stoke-Wolverhampton-Birmingham is essential. It is good to get to Birmingham before an AI player does (if there are any), but if not, just build another station and keep going.
From there I concentrate on building up (and buying up) the textile industry in the North, connecting to Shrewsbury (coal and/or iron) and then on to London. London-Birmingham and then London-Bristol are very profitable routes. Hereford usually has coal coming upriver, and Cardiff is a great place to find coal, as are some cities on the east side (Newcastle-Leeds area).
You must build or buy industries as needed, or service existing ones if you can't buy them. The textile industry around Manchester and Bristol will pay large dividends, as will the sugar/alcohol trade from Liverpoole to Worcester and Bristol to Slough. Building up a lumber and pulpwood chain out of the eastern ports (Southampton, London, etc) will also pay. Owning the industries (since the game doesn't always build them where you want them) pays extra.
Not sure where your cash-flow problem is, though there is an intentional kink around 1840-45 as your first locos come up for replacement and the Battle for the Gauges comes down to the finish. I am usually awash in money by 1850, but I do know all the ins-and-outs of the scenario. I would encourage you to try again, with emphasis on building up the cargo chains for cotton-cloth, sugar-alcohol, logs-lumber, pulpwood-paper, owning the industries where possible and expanding them as needed. Above all, get your coal haulage above 50 loads per year as soon as possible.
British Miracle requires you to win the Battle of the Gauges, but it isn't just about expanding track miles. The railroads need functionality - hauling raw materials from the ports and finished products to them. Expanding the economy is the real purpose of the scenario...
I'm sorry you had a bad experience in your first game. I hope you'll try again; I think British Miracle has high replay value.
Feel free to PM me or post here with questions or comments.
Any thoughts on other events or goals that should be included? Is the scenario difficulty level about right, or should it be easier or harder?
(I wonder how many know what I'm talking about? 
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