If you have proper blank number plates made of the correct material and carrying the British standard mark (BSAU145d) in the bottom right corner, then they should be legal. If you have stuck or printed the registration number ont a random piece of yellow board or card, then no, they will not be legally compliant.
To be legal, the plate has to carry the registration in the correct font, with the digits of the specified size. The digits must be black/dark and non reflective, and the background must comply with the specifications laid down in BSAU145d, which includes that it must be retro reflective (white for the front, yellow for the rear), and must be able to distort to a specified amount, and return to its original shape amongst other things. The plate must carry the BSAU145d imprint at the bottom right corner to show that it complies, and must have the name and postcode of the licensed maker or issuer clearly marked on it.