Available from OTTD's content browser in-game under the following names:
Scenario: United Kingdom: 1860-2100
Heightmap: Flatter UK & Ireland, 2048 x 20
(Both names are a little wrong and I can't change them, but it doesn't really matter.
A scenario I made for myself, but shared here. Now more modifiable since v1.1: You can edit the NewGRF selection (at your own risk!) in the Scenario Editor by typing "set scenario_developer 1" in the developer console and loading my scenario into it. You should keep the town and bridge related NewGRFs, and demolish the two existing default industries off the map (1 coal mine, 1 power plant located between Southend-on-Sea and Maidstone) before adding a different industry NewGRF, but everything else should be "safe" to change.
for more info.
---United Kingdom - 250 Years Scenario
A different take on something that's been done before a few times by others. The scenario is set in the UK & Ireland and is designed to be fun to play and give a paced endurance challenge without being too complex.
Play over 250 years between 1850 and 2100 - Since you have a long time to play, there's a slow start, and you have to work to build your transport empire. The UKRS2 newgrf provides trains until 2020, when default Maglevs and later Vactube trains will appear (but Monorail won't) until 2080-ish (if you get that far).
Design a network to link and grow the towns - Passenger routes may not make much money with the high running costs of trains, so you should subsidise your passenger routes with industry routes. Only a single (default) coal mine and power plant exist on the map to get you started - But many more will spawn randomly soon after you start playing.
The map is large with only a small landmass - I've set the industry density to "very low". Although the game will cram very many onto the land anyway, new industry construction reaches it's peak at around 20 years in.
I found that existing heightmaps of the UK were too hilly for my liking, so I made my own in Photoshop by tracing various maps, then painted on mountains over an elevation map using a brush type, giving them a rough look (unlike the smoother hills of Genesis world gen), while still being flat enough to easily build over, since they're not higher than 4 tiles above sea level.
Towns were chosen based on location, distance from other towns, real-world population, and historical importance - Which is why some large towns in the south of England don't exist in this scenario, but yet smaller towns in Scotland and Ireland do. All but the major cities have low populations for the player to grow. England has many more towns than Ireland and northern Scotland to reflect real-world population distribution.
They were placed by eye with maps as reference (some may be in the wrong place - The heightmap came out skewed in a weird angle) starting with those closer to the coasts, then working my way inland using the coastal towns as reference points. City towns in real world got the "city" status in-game, even if they became cities long after 1850.
It took a long time to build this scenario and I often left it to run while going AFK - Trees have run rampant across the map even though I clicked "Random Trees" only once before building any towns (big mistake!). You can make them transparent, but towns may hate you for a while when you build near them. Start with stations first before building anything else.
The "Progressive Rail Set" Newgrf is included so that Maglev trains can run on VacTube rails during the very late game (if you get that far), since VacTube trains cannot carry cargo aside from goods. This is why I've set vehicles to not expire (MagLevs do at around 2060 for some reason!), but old vehicles can be hidden in the depot menus.