Marketplace + bank double the ammount of a trade route at the very least ! Very interesting !
Marketplaces and banks have no effect over the amount of trade a city receives from a trade route, they only increase the luxury and tax output in the city, so they only affect part of the trade, and only after it has been received and allocated to luxury and tax. The amount of trade received fluctuates for other reasons--cities can grow or shrink, governments can change, etc.
In civ1 water tiles arent that good
I disagree. They aren't as good, but they're very good overall. Yeah, they can't produce shields like in Civ 3, but in Civ 1 there's also no shield waste. And 3 trade per square (4 with the Colossus) is a powerful incentive to work as many sea squares as you can. In fact, when fully grown, my biggest trading cities will often work more sea tiles than land. With fertile grassland or rivers and some fish, you only need 5-6 land squares to feed a city of 20 that will generate roughly 50 arrows
before accounting for foreign trade. This baby doesn't need production. It can buy its way into space.
Which brings me back to the OP...
After all I've said, I find the city placement here slightly sub-optimal. Birmingham could've been built on the tip of the peninsula, one tile to the northeast of where it is now. That would've accounted for more trade (3 more sea squares) and made room for a bigger city to the west. But even with the current placement, if you cut down the forest, railroad the square, build on top of it and fully develop all land in range (turn the horses into forest for 1 more food), a city there can work all of its 9 sea squares and only the mountain would remain unused. Also, a city in the plains north of the London-Coventry railroad, could potentially generate 8 surplus food from its land tiles and the fish, 9 if you also irrigate and railroad the hills. This would allow it to work 5 more sea squares and 4 irrigated desert tiles for optimal trade. Both new cities will max out at size 13 and generate 7 shields--more than enough to meet their defence needs and leave a handy surplus for rushing the construction of those markets and banks. For me, this small isolated continent is a game-winning starting position.