BTEC for ucl

I’m doing A-Level/Btec, would doing a btec put me at a disadvantage?

The fact that you haven't mentioned what type of BTEC(s) you're studying (e.g. Extended Certificate, Diploma, Extended Diploma, etc.), the subject (e.g. Applied Science, Business, etc.), or the course at UCL to which you'd be applying, makes this question impossible to answer with any accuracy.

Generally speaking, UCL only accept BTEC Extended Diplomas (not other types of BTECs), and not all courses even accept those. (Search for BTEC on UCL's Entry requirements page, here, for confirmation.)

I do law, extended diploma in crim, and level 3 extended certificate business, is that fine?

When you say "I do law", do you mean that you're applying for a Law degree? Or do you mean you're doing A level Law?

When you say "extended diploma in crim", do you mean the WJEC Level 3 Applied Diploma in Criminology? (There is no BTEC in Criminology, unless you mean Forensics and Criminal Investigation.)

Knowing exactly what you're studying is incredibly important. UCL specifically say on the same page I mentioned above that:

"The WJEC Level 3 Applied Diploma in Criminology is only accepted for entry to the BSc Crime and Security Science programme, and the Engineering Foundation Year programmes. This qualification is not accepted for any other programme at UCL."

What course we're you hoping to apply for at UCL?

WJEC level 3 applied diploma, Applied law (Extended certificate) and I was planning on doing the engineering foundation year.

OK. Those qualification should not be an issue as the Engineering Foundation Year programme is designed for students who do not meet the standard entry requirements for UCL’s undergraduate engineering degrees. Not having done A levels will not put you at a disadvantage in this case. (However, not having studied anything remotely to do with engineering might, as it may make it difficult for you to convince them that you "aspire to become an engineer" - i.e. that you meet the profile of their target student. Your personal statement will have to be very persuasive with regards to your passion for engineering.)

You can see the full set of eligibility criteria for the Engineering Foundation Year programme here. Do check it carefully as you must also meet non-academic criteria.

For example you must meet one of these criteria: Care experienced, Forced Migrant, or Mature Student.
Or you must meet two of these criteria: Young Carer, Estranged, Educational Gap, Eligibility for Free School Meals, Postcode Eligibility.

(UCL's definition of those terms in on the page linked above in this post.)