It's not about doing 100mph and winning races off the traffic lights.
If you have a car with an engine that is underpowered for the type of car, the outcome is not only that it'll be a bit slow. On many occasions, it'll need to be worked hard, harder than if you had a car with an engine that was more suited. A prime example of this is people who find something like a 0.8 litre Matiz is not as economical on the Motorway as, say, a 1.4 litre Fiesta.
It's also a more stressful drive - a lack of torque means you need more effort to do pretty much anything, and an obvious byproduct of having to work the car harder is that you will consume more fuel.
This is why when buying a car its important to match the engine to the car. Nobody is suggesting you should have gone and bought a 135i with 306bhp instead - if you dont need performance, great, avoid the performance variants.
But something like a 118i or even a 120i are not performance variants. They have a similar engine to the 116i - infact it's the same M43 unit only with different power outputs and capacity - but because the power is more suited to the car, its a more relaxing drive, you work it harder, and as a result...
.... they are more economical.
I've driven every single engine in the 1 Series range bar the 130i and the 135i and by far the worst was the 116i. It was simply diabolical and I'm not saying this simply because it didnt have the 230bhp I am used to. It was barely, if at all, fit for purpose. The marginally larger petrol engines were much better and in my experience were noticeably better on fuel as a result.
£30 a month in the context of buying and running a £12,000 car is not a large figure. If it *is* a large figure then its probably sensible to ask yourself what on earth you are doing with a £12,000 car in the first place when a £6000 car would do much to alleviete whatever financial position you've got yourself into where 30 quid at the end of the month is noticeable.
And then there is the other side of the financial coin - depreciation. The 116i is the runt of the litter. This is less of an issue when its newer as different types of people buy them new compared to when they are 3-8 years old on the used market. But as it gets older, and by the time you want to get rid of it, it'll be worth considerably less than a better engined model simply because very few people on the used market will want a 116i. You may find this ends up costing you more money than you've saved.
NEVER only look at the monthly cost of owning a car. It's a very, very flawed way of thinking.
And I'm sorry but to say you wanted something 'more sporty' and to then buy a 116i is second only in comedy to the idea that the reason I don't like your 116i is because I am jealous. Jealous? Of what?! The reason I don't currently have a car worth the same as your 116i is not financial, thats for sure!