I did a BTEC National Diploma in Business management and Marketing. I myself wasnt to sure if they would allow this, although it was absoloutley fine. My AFCO said they say that about BTEC's because there are some courses that arent as advanced (joking there were ones probably about ball point pens lol), and therefore in a way would not justify the UCAS points. Although I think engineering would be suitable as it is quite an advanced course. Your AFCO should make a quick phone call to ensure that it is suitable and also double check your points, mine were a little different to what I origionally thought.
The full UCAS points tariff, showing equivalent qualifications, is available at UCAS.com
You cannot add AS & A level UCAS points in the same subject & the subjects must include at least two non-overlapping subjects scoring over 45.
The grades that count are:
A Level/Advanced Highers: A=120, B=100, C=80, D=60
AS Levels: A=60, B=50
Scottish Highers: A=72, B=60, C=48
BTECs Certificates are only valid at level 3 (Distinction, Merit grade pass or better).
BTEC Diploma (Merit, Merit, Merit)
All BTECs, HNDs must be verified by the Admiralty Interview Board as accepted non-overlapping subjects of sufficient academic content.
Some courses do not have sufficient academic content & are therefore not accepted.
....To cut a long story short, in answer to the initial query & as stated by Drakey earlier, those failing to meet the minimum acceptable academic threshold are most likely not going to join as an Officer.