I applied to four universities this year (including UCL) - two of them required a portfolio to be sent and UCL sent an assessment task that was also art-based. I think the majority of them also preferred an A-level art qualification, and as the person above mentioned I found that the majority of the work I put in my portfolio came from previous A-level work. I took maths and physics alongside art, which worked out fairly well in my opinion as there was some overlapping of content (particularly in the mechanics modules) that lessened the workload a little, however they can both be quite difficult. Honestly overall I'd recommend taking art as a full A-level and only choosing two others as I've known people to drop down from three to two A-levels, so four is only an option if you're really sure you can handle it.
Also I got offers from the three other universities, but I should warn you that the admissions process for architecture at UCL can be very slow - I applied around 15th December last year and besides sending the assessment task mid-January I never heard anything back and had to withdraw my offer at the start of March in order to confirm a different offer (apparently it could've taken a couple more months than that). If you're applying by the October deadline (I assume so if you're applying to Cambridge uni) then you might not have to worry about this, it may only be an issue for later applicants, but I thought I'd warn you just in case.
One last thing - it might be a good idea to find out what each uni's course is like - the one I've settled for (BCU) is mainly art-based(?) but a lot of the work revolves around 'live projects' and such, whereas none of the other courses I applied for mentioned this. I think courses vary between art-based and technology-based from what I've heard (I might be wrong here) so depending on what the unis you want to apply for offer, it could be a good idea to base your options on this. I'm fairly sure UCL is also more heavily art-based - none of the courses I applied for were 'technology based' and I only heard this term from one uni (Kent) that claimed it was a fair mix of both so honestly I don't really know what it means, maybe whether they lean more towards traditional or digital methods?
Anyway sorry this is so long, hopefully some of this info helps!