I'm not sure what you mean by a "backup" but I'd absolutely recommend you do what you want to do, not what your parents want you do. I've just finished AS maths and am carrying it through to A2 and if you don't like it as GCSE I would recommend it for A level. There's a *lot* of emphasis on algebra and graphs so if you didn't enjoy those at GCSE, you're not going to enjoy A level.
I never did classics or economics myself but have only ever heard good things about classical civilisation from three of my friends that do it. There's a lot of essay writing and sources and things, much like GCSE, but just more of it, so I've heard
With your university choice, that'll probably change at some point and may change back. When I enrolled at my sixth form I said my intention was to study physics at university, failing that English Language. Within two weeks, I dropped physics and I still don't enjoy English and will be applying to study psychology at university instead, so it's good to keep an open mind about these things but I see no reason why you'd need a "backup" university option.
So long as you're getting the grades you need in assessments and such like in your languages, and they reflect the entry requirements of the university(ies) you want to study at, you'll be fine. You'll do your university research/personal statement etc. next summer and send your application off winter 2017. There's really no opportunity to change subjects as soon as your application is set in motion (given you only get one personal statement for five choices) unless you delay a year and reapply for 2019 entry instead so I'd just say go for the languages.
I assume you mean your parents want you to have maths as a backup option so you could pursue that at university of languages don't work out, but if you want to apply for the top 30 or so universities in the country, an A level in further mathematics is an absolute must or you'll be severely disadvantaged. Some universities I've seen won't consider your application unless you have maths AND further maths. I considered studying it when I dropped physics but decided against it for this reason.
Hope this helps, and do what you want to study, not what your parents think you should
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