Can I still go to a decent university

I would say you can get into places such Oxford Brookes or University of East Anglia, Essex, Manchester Metropolitan, Liverpool John Moores etc.

Based on what exactly?

Most 'top' universities, including Russell Group, accept BTECs and a whole range of non-A level qualifications.

Well, most people regard as 'top' places such as Oxbridge, LSE, KCL, UCL, Durham Edinburgh, St Andrews. Then you would see Nottingham, Manchester, Sheffield, Birmingham, Bath, Bristol, Exeter. Then 'low tier' RG such as Liverpool, Southampton or places such as Lancaster. Below this, some of the unis I previously mentioned. Most if not all top RG and top unis do not accept BTECs. Some of the low tier do for less competitive courses. Nevertheless, most people won't have such qualifications even there. You would find that BTECs are more common in unis that often able their admission process on UCAS points such as Brookes or UEA.

Most if not all top RG and top unis do not accept BTECs.

I’m at sixth form doing 2 A-Levels (English lit and media studies), 1 Level 3 (business studies) and re-sitting my maths. Because I’m not doing 3 a levels can I still go to a decent university?

They do accept BTECs - both Extended Diplomas and 'one subject' BTECs in combo with A levels.

(This is the problem with 'I've heard' - its actually 'I assume', and its usually wrong.)

Well, as I am not excluding that you can get into places such as Liverpool with 1 BTEC, but UCL, KCL, LSE, Durham and Oxbridge surely do not.

A quick google shows me that UCL https://www.ucl.ac.uk/prospective-students/undergraduate/application/entry-requirements/other-uk-qualifications and Kings https://www.kcl.ac.uk/study/undergraduate/apply/entry-requirements accept BTECs. I've not checked for any other universities but it shows the poor advice given by those who assume without checking.

Well, as I am not excluding that you can get into places such as Liverpool with 1 BTEC, but UCL, KCL, LSE, Durham and Oxbridge surely do not.

My bad then. Good luck applying to Medicine or Law to such unis with BTECs.

Well, most people regard as 'top' places such as Oxbridge, LSE, KCL, UCL, Durham Edinburgh, St Andrews. Then you would see Nottingham, Manchester, Sheffield, Birmingham, Bath, Bristol, Exeter. Then 'low tier' RG such as Liverpool, Southampton or places such as Lancaster. Below this, some of the unis I previously mentioned. Most if not all top RG and top unis do not accept BTECs. Some of the low tier do for less competitive courses. Nevertheless, most people won't have such qualifications even there. You would find that BTECs are more common in unis that often able their admission process on UCAS points such as Brookes or UEA.

Durham does : https://www.dur.ac.uk/sociology/undergraduate/bacriminology/entry/
UCL does : https://www.ucl.ac.uk/prospective-students/undergraduate/application/entry-requirements/other-uk-qualifications
So does KCL : https://www.kcl.ac.uk/study/undergraduate/apply/entry-requirements/btec
And Oxford : https://www.ox.ac.uk/admissions/undergraduate/courses/admission-requirements/uk-qualifications
So does Bristol, even for Medicine and Law.

Perhaps you might like to adjust your ideas about the academic value of BTECs and stop jumping to conclusions.

And Oxford : https://www.ox.ac.uk/admissions/undergraduate/courses/admission-requirements/uk-qualifications