"...since I will have to pay the tuition fees from my own pocket..."
You want to pay for an Oxbridge degree AND self teach?
"However I am not going to commit 3 years of my life and tuition fees+oppertunity costs exceeding £40000+"
So you want to pay for an Oxbridge degree AND self teach AND do it full time?
Unless you're rich, you're just making life difficult for yourself. But if you're rich... Woah!
But if you're not?
There are loads of options available, I just had a gander at Google...
Open University:
BSc Mathematics (accredited)
~£16,000 for a complete degree
Excellent books (particularly for autodidact-types)
Flexible study arrangements (course can be completed in ~3-15 years)
Tutor support should you need it
Personally, I'd take a guy with BSc Mathematics from Open University than some Oxbridge toffee. Again that's just my 10 pence (devalued currency, THANKS BREXIT!).
"If universities really are boosting their entry requirements above what they would accept just how felixible is it?"
If you're not getting A*A*A*/AAA you can forget about Oxbridge, they're spoilt for choice. No use complaining about grade inflation either: everyone wants a degree of sorts and a job that doesn't involve menial labour crap- this applies to EVERYBODY. There are hundreds of thousands of students, a quarter of A level grades are A or above, statistically speaking there will be no shortage of candidates with AAA or more.
I wouldn't worry too much though as there are plenty of non-Oxbridge institutiones that are great, and also: universities want £££ so there will be no shortage of options available for you. I suspect this is not the answer you want, you're more interested in prestige aren't you?