These new cars that have rear number plate lights that are LEDs and are bright.

I have just enquired at EuroCarParts about getting them for my car, I was told they are illegal to use them on road, (for off road use only) how come the new cars that have them fitted are allowed to use them on the road.

Bull mate.

As long as they point down and not outwards.

Should be ok if replacing a fillament bulb in original position so as not to show a white light to the rear, and that they are white, and that they have an approval mark. To quote -

"(i)a marking designated as an approval mark by regulation 5 of the Designation of Approval Marks Regulations and shown at item 10 of Schedule 4 to those Regulations; or
(ii)a marking designated as an approval mark by regulation 4 of the Designation of Approval Marks Regulations and shown at item 4 of Schedule 2 to those Regulations."

A lot of cheap LED lamps don't have this, as it costs to get them through approval etc., but you can get approved ones ('e' marked) for most applications. It's very unlikely that you would have someone check the bulb for markings, but if they did, it could mean 3 points on your licence for having the 'wrong' ones, even if they came from the same production line.

The regulations that apply are in the Road Vehicle Lighting Regulations 1989 (as ammended). See RVLR1989