Completing a single A-Level in a year.

I got ABD in politics, economics and creative writing. This means that the creative writing grade is properly messing up my application, so I've contacted my 6th form and they say it is possible to just do a single A-Level for the year, (besides I was already taking a gap year). I'd be aiming for an A ultimately, so I was just wondering how realistic is this expectation and what courses would this likely be achievable in the single year? I was thinking of picking up either History or Philosophy.

It's not the most common thing, although not wholly uncommon depending on your school. At my former further education provider, it was not uncommon for students to undertake a "year 14" - either to pick up subjects they realised they required and hadn't taken, or to retake one or two subjects. At least one person I know did two and a half subjects - although part of this was Maths and Further Maths, following IB HL Maths - so a fair amount of overlap in core material, albeit with some additional applied and further material not on the HL syllabus (at the time).

It's perfectly possible, whether it's achievable in your case depends entirely on yourself. But with good time management and study technique I see no reason it wouldn't be achievable.

So I have done both Maths and English Literature in a year. My advice would be that something like Maths was ridiculously easy in that year compared to English Literature, if you are going to be cramming an A Level into a year it is best to go for something that requires technique rather than a massive amount of memorisation.

I took Politics in a year and got a B. In my breakdown, I got 2 Cs (both 2 off a B), a B, and a high A so it worked out as a B overall, though I can't be bothered getting the Cs remarked as I'm happy with a B.

It's not easy, for sure, but it depends on how much effort you're willing to put in. Because politics was a modular course and AS was UK vs US as A2, I treated UK and US as totally separate subjects so, in my mind, I was taking 4 A Levels but only getting 3 results.

I would look at the two courses and see which one is A) more interesting and B) easier to do in a year. There's no point picking something where people studying it for 2 year only manage to scrape Cs because of its difficulty and don't pick something that you know you're going to hate as you'll just end up regretting it and not doing well anyway because you don't enjoy studying it.