The catch for the bonnet may have failed/ been "sticky" due to lack of use/ lubricant, corrosion, or a broken spring, it probably appeared fine untill you were travelling at high(er) speed, when the air pressure got under the front edge and lifted it off, once it was up it would have thrown it back with some force

it is part of the MOT and i feel you have a good case for blaming the garage

i would approach the Citizens Advice Beauro, and trading standards, and speak to the garage, it seems obvious not to shout, swear or loose your temper when speaking/ dealing with the garage, but it can be hard for many people to do this, keep calm and assert yourself, they will probably say it is nothing to do with them, because they will not want to pay out money/ do additional work - so you may have to be persistent with them

If the garage refuses to fix/ pay for the car being fixed, its worth negotiating with them, perhaps a refund for the work or even for the labour, something is better than nothing - as this is an MOT related thing, you could explain you may contact VOSA, or it would make an interesting story in the local press "local garage MOTs death trap" etc - it could have caused a serious accident

i recently replaced my winscreen due to a split for the MOT, i had to get fully comp, and it was a £50 exess on the screen (so my new screen cost £50) , i was told it could have cost up to £600 with it being a heated screen,

if you are fully comp you could claim off your insurance, your insurance company would probably try to recoup the cost from the garage, if you had a very high exess (eg- £250) and your windscreen exess was only £50, it may be more cost effective to claim on only the screen, and pick up a secondhand bonnet and fix the roof yourself

I know very little about consumer rights and the law, i am just thinking aloud and putting it "out there" - so i would contact those that do! hope you get it sorted and good luck!