I can only really comment on the UK application system, I have never had anything to do with US universities.
Warwick has a pretty good reputation for maths. Only you know if it's a good research match for you, however. If you're fully funded and interested in the research being done there, I say go for it! I considered applying to Warwick too, but didn't find a good research match for me there. Also, don't hold your breath for Cambridge, they very rarely admit anybody to the PhD in maths directly, they usually make you do Part III (the last year of their four-year masters' course) first and then admit PhD students according to their Part III results. That's why I didn't even bother to apply there. (Got a fully funded offer for Oxford now though, so no regrets. )
About waiting for another year - well, you'll be a year older, but will anything change to your application by then? It does seem weird to me though that only so few of your applications were successful, considering your excellent background. (I don't know what GRE scores mean though, not familiar with the system, but that 26% in there doesn't look great?) Anyway, if there is room for improvement in your application, it might be worth waiting a year, but really, Warwick is pretty good already and I'm not sure if the chance of going to a slightly more prestigious university is really worth wasting an entire year.
You seem to have applied to a lot of schools - did you really put any thought into your specific applications, or just applied to any school with a prestigious name? Because that might be part of the reason why you didn't get in / didn't get funding. For example, in one of your other posts (I think it was over at TSR, not at grad café) it became clear you didn't know how the college system in Oxford works, how you will be assigned a college, and you didn't find out about possible college funding opportunities before submitting your application, therefore potentially losing out on some important funding opportunities.
Familiarize yourself with the application process and system at every university you apply to. Only apply to universities you're genuinely interested in attending, not just any university with a big name. Your application materials, background, statement of purpose, research proposal etc should match the programme you apply to, and at your interviews you should be able to make clear why you chose to apply to this particular university / research group. That might be something to focus on some more if you decide to wait another year and apply again.