Hey, It is definitely possible. I didn’t do further maths but I did make sure I at least taught myself the AS content and sat the AS exam, you can still do it without even the AS but the unis will ask for higher grades in other subjects.
Like if the standard offer is A*A*A which I think it was for Bristol then they might ask for A*A*A* with a good grade in one of the Oxford or Cambridge entrance exams.
I didn’t go to a Russel group but I did go to Bath which is ranked higher in maths than most Russel groups and I got offers from all the russel groups I applied to (other than Oxford ☹️), these were: Bristol, Southampton and Exeter.
It’s definitely possible, your personal statement should express your enthusiasm for maths at a very super curricular level, they want passion because otherwise you won’t make it through a maths degree just being “good at maths”.
I will say, however, that I’m quite concerned that you’ve said it’s hard to teach yourself further maths. Because at uni you will find that the ability to teach yourself maths far more complex and abstract than Alevel further maths is an essential skill, so if you are struggling now you must really ask if Uni maths is the right choice.
I don’t know what the deadline is so but you absolutely must apply to sit the MAT or STEP paper ASAP, your school will usually host this. STEP has further content so the MAT will be easier. But it Is absolutely your best bet at getting on a competitive maths course without further maths, most unis will ask for a good score to make up for the lack of further maths so without both your chances are small.
Good luck!