The "Level 3" part of the qualification name tells you it's the same complexity / difficulty as an A level. I was asking about it's "size", because some people take Extended Certificates (worth 1 A level) each and some people take an Extended Diploma (worth 3 A levels). There are also Certificates, Diplomas and Foundation Diplomas - all of which are BTEC Nationals Level 3. However, the fact that you're asking about "other subjects" suggest that you planning to do three BTECs, which suggests that they are indeed Extended Certificates.
Bristol would have potentially considered you for their MEng Aerospace Engineering course without a Maths A level, but only if you'd been taking a BTEC Extended Diploma (worth 3 A levels) in Engineering, rather than three separate BTEC Extended Certificates. "
D*DD in either Engineering BTEC National Level 3 Extended Diploma, with Distinctions in four required Physics units, plus A in Mathematics at A-level (or equivalent), or in Applied Science BTEC National Level 3 Extended Diploma, with Distinctions in five required Chemistry units, plus A in Mathematics at A-level (or equivalent).
Applicants taking Engineering BTEC may be invited to take the University of Bristol mathematics test in place of A-level Mathematics." (
source)
University of Central Lancashire don't need Maths A level either, but like Bristol they want you to have done a single Extended Diploma in Engineering. "BTEC Extended Diploma: DMM in Engineering including Maths units" (
source)
The same for
Kingston: "Alternatively, BTEC Extended Diploma in Engineering or related subject such as Aerospace/Aeronautical/Electrical/Electronic/Manufacturing and Mechanical Engineering will be considered (grades DMM)." (
source)
And
Brighton: "BTEC Extended Diploma DMM. Must include further maths for engineering techs." (
source)
And
City: "D*DD in Engineering (First teaching Sept 2016) with minimum grade D in units 1 - Engineering Principles, 7 - Calculus to Solve Engineering Problems and 8 - Further Engineering Mathematics."
Nottingham Trent too: "We accept the BTEC Extended Diplomas below. If you are studying a different BTEC Extended Diploma you will also need a suitable level 3 Mathematics qualification.
Extended Diploma in Engineering
Extended Diploma in Electronic Engineering
Extended Diploma in Mechanical Engineering." (
source)
It certainly seems that without A levels maths, you'd need a BTEC Extended Diploma (worth three A levels) in Engineering, rather than three separate BTEC Extended Certificates (worth 1 A level each). Having said that, you could try contacting some of the above and asking.