I can guarantee that is asbestos 100% no doubt about it, it's most probably chrysotile (WHITE). Most unlikely to be blue or brown which is the most toxic. White is considerably less toxic and breathing it in as you have done will NOT cause irreversible damage to you.
Asbestos is in the air we all breathe, it's in the environment in very small amounts. The issue with composite material is that there is obviously more of it in concentrated amounts but breaking or snapping it in places (considering it's white) WILL release fibers but really it's NOT a big deal, you haven't been there all day sanding it down so no need to panic.
I'm not going to get into a debate about this with others, but white asbestos is broken down in the lungs, it takes a long time and it is a carcinogenic - the difference is the shape of the fibers. Brown and blue fibers are metallic and lodge themselves deep in the lungs where the lungs simple cannot remove it. White has a weaker structure and is more susceptible to the lungs natural washing process.
FIRSTLY - DON'T PANIC
Take and send off a small sample to a lab for testing, this shouldn't cost more than £15, they will be able to tell you what type of asbestos it is. Definitely get confirmation that it is WHITE and not blue or brown. In the meantime try to avoid working in the immediate area.
SECONDLY
Find the HSE guidance on asbestos removal and follow all the recommendations, wear a mask, seal the room, clean up, bag and dispose etc in accordance with instructions - you probably won't find guidance for removal of asbestos sheeting but you could apply the same procedure for disposing of asbestos artex or ...
IF
You're not comfortable with this, phone about and find a LICENSED asbestos remover and pay for it. It won't be cheap BUT you'll have peace of mind. Licensed contractors have access to better equipment usually, they put the room under negative pressure, etc that sort of thing, which you can't do if you do it with a hoover lol. They'll also be able to take a sample on your behalf if you don't want to go near it again.
So those are your options...
BTW - before anyone quotes me saying you can't tell what it is by looking at it... Yes you're right, but given the location of board and it being a residential and also the appearance of the fibers it's very unlikely to be blue/brown although like everything there is always a chance, best he gets it tested