Well done for recognising the need for a back up career in case it all goes wrong. It frequently does in this game and it is vital to have a plan B up your sleeve - many people overlook this point. A marketable degree in useful subject is always good to have, but it will take several years and cost quite a bit of money in itself. Alternatively, training as a plumber or electrician will always give you a reliable source of income.
Regarding A levels, Physics is quite useful for understanding some of the concepts you'll study for the ATPL exams, but Maths isn't really necessary. I failed Maths myself and it doesn't stop me flying a Q400 - the only maths I use day to day are adding, subtracting and the three times table - seriously.