The entry requirements are not the same, and the time spent doing a HND is 2 years as opposed to 3 years with a degree.
When I was studying for my degree the people who had spent the 2 years doing their HND was accredited the 1st year of a degree, So some students in my class in my second year has already been there for 2 years and got a HND.
I was also told if I dropped out of Uni after my first/second year I'd still walk away with a HND.
If you don't get the grades for a degree go for a HND as it'll be a good back up should you not land a Pilot job, I'm sure you could always do a top up course and spend another 2 years before getting a BSc/BEng in Aeronautical engineering.
And as CG said, get in with the Uni Air Squadren and you have some good flying too (I think you'd be able to get in the UAS with a HND? ... Might be worth checking though).