Oh my god - you could be me!! I have ds1 who is 3 and was 7lb 10 at birth. I now have ds2 who is 3 months - I knew from the first scan that he was small, I also had numerous growth scans at which I was told he was still growing but still small!
Eventually he was delivered at 38+5, weighing just 4lb 15! He was absolutely fine, no problems other than being small. I was determined to breast feed, knowing that being small he needed all the help he could get with the immunities and extra benefits of b/f.
Nobody tried to force formula on him, although it was mentioned on the first night as he was slow to start feeding properly. I persevered with getting him to latch and he soon got the hang of it. When formula was mentioned, I said I would prefer to express and feed him that from a cup or spoon rather than give him formula and the midwives were happy to try that.
He's putting weight on very quickly - about half a pound a week and now weighs a healthy 10lb 4!
I've heard many people say that for small babies actually it's even more beneficial for them to b/f rather than sticking them on formula to try to fatten them up, so you may find that the midwives are more patient and willing to make sure b/f is successful for you.
Having said all that, your baby may not even be small! But I hope I've gone some way to putting your mind at rest, and best of luck with everything
!