2 things
1. Maybe rules are different in the UK, here it is not legal as the car was not approved to use LED bulbs, if the police stops you, they will fine you and you will be asked to do a MOT and the same, if the MOT station sees these, they will also fail you.
2. The big question regarding if it will work is more related to how the car actually feeds the bulbs. If its through a simple relay, it will probably work, but if not, it might fail as the LED draws significantly less power so the car might think the bulb is blown and cut the power. This can be remedied with a loading resistor to make the circuit use as much power as the halogen bulb.
If memory serves, i saw someone replacing some of the rear bulbs and they had to put in resistors to get it working.
I know people that have replaced their halogen with LED, it mostly gives off better light and no one is flashing them, but they do know, if they get stopped by some officer that is being really an*l they risk a fine.
PS. There is a ton of different LED bulbs out there, they are not all made equal, if you buy the cheaptest ali express bulbs they might not be very good