Well actually it's ONLY legal to ride with flip up part in the non locked down position if the manufacturer has sated it is....
Some are designed to have the lower front section flipped all the way back over the helmet, some allow the face bar with visor to flip up to the top of the helmet...these are NOT in general supported by the manufacturer to be in a safe configuration to ride with in the up position...
All started off when the police Mcy riders were told not to ride with them open for insurance liability issues firstly and safety issues later...with some riders complaining of neck strain when riding at speed due to the wind drag...and buffeting...well doughhh...
If your manufacturer states it's legal to use in the UK in the open position or with the race bar removed then it should be ok...but carry the instructions with you as police will probably stop you....if the face bar is raised up...
PS my own opinion of seeing the resulting injuries to riders with open face helmets hitting the road at speed is...... Only buy a full face helmet...the face part is designed to protect your face... An open face helmet is NOT... think about it...
Warning following actions will result in injury and or fatality. Do NOT try it...
Just think about what it would be like if you did...
Put a large orange in your hand and scrape it along a brick wall...
Then try the same thing but put your fingers between the orange and the wall and try to scrape through to the orange...
Put the orange in an old helmet and scrape that against the wall.. Note the difference in pain, blood, stinging etc...
Put a large orange in your hand...run forwards and the jump/leap/dive forwards allowing your outstretched hand with the orange in to take the force of your fall..try and get the orange to hit the road surface first....and scrape along the road for the 7-10 inches you will slide...
Then if you feel up to it put the orange in your mouth, tie your hands behind your back and run forwards again..diving/leaping forwards so your face hits the road surface this time...
No...don't want to do that? Well just think of doing it at 15-30-50-60 mph...
So why would anyone really want to wear an open face helmet..