The standard is for A-levels to be taken in two years. If you spread it over three years, some unis (elite of the elite, and strong unis) would be concerned you didn't have an intense enough workload for your 6th form studies, and you wouldn't cope.
It's not just the level of content - it's also the pace. That's why they're concerned. Besides so many uni applicants would do their A-levels in two years anyway.
https://warwick.ac.uk/study/undergraduate/apply/admissionsstatement/ Warwick's admissions statement
They don't object to resits within the normal 2 years. It seems they are concerned with the pacing (2 years is standard for A-level). Other unis like Oxford and Cambridge are also concerned. Just bear in mind this.
If you have 3 years to prepare for the same a levels, you'd be at a significant advantage compared to those with only 2 years to prepare. Hence, unis adjust for this.
If you attempt to do GCSE a year early, then start A-level content at Y11, then you'd have to finish
all the A-levels within 2 years.
Warwick: "The University normally expects applicants to demonstrate that they can succeed on a demanding course of study within a defined timescale, as exemplified by (but not limited to) the achievement of three A levels (or international equivalent) over the course of a maximum of two years of study."
3 A-levels over the maximum of 2 years. If you start A-level math in Y11, you'd need to finish it, along with 2 other A-levels, by Y12, to meet this requirement. Effectively you'd be skipping a year in that case. Some people do that.