I am a former admission tutor and I agree with @HPFA to some extent, especially because DC is willing to do a gap year if this doesn’t work out.
However the main problem is that decision time is a long way off. It may seem now like it will be easy to start planning for a gap year if this doesn’t work out, but reality may hit very differently. Every admissions tutor has been the recipient of heartbreaking pleas from applicants who regret this strategy in August.
A good back up programme is unlikely to be able to accommodate them unless its recent history suggests otherwise.
Secondly, if gap year planning starts late, many of the best opportunities, the ones that would enhance a repeat application, are less likely to be available. How does DC plan to improve their second application to the same programme?
Finally, if this is relevant, if Maths A level is required a number of STEM programmes discourage a gap year from a concern Maths will be lost. (I don’t agree but there it is).
Why limit yourself this way, when you could simply reject the alternative(s) later?