Small number plates are an advert for the police to stop you in your tracks. It is an absolute offence to have a number plate which does not comply with the regulations.
There are strict rules relating to spacing under the “Road Vehicles Display of Registration Marks” Regulations 2001 which came into force on the 21st March 2001. The registration plate must be fixed vertically to the rear of the motorcycle, be distinguishable in normal daylight and be properly lit.
You are not allowed your registration on a single line and character height must be at least 64mm. The DVLA have the power to take your registration mark away from you if you are consistently reported for having a defective number plate, which can mean that your motorcycle would be left with a “Q” plate substantially reducing its resale value.
If you have a classic motorcycle, different regulations apply.
If you put anything on your number plate which is designed to reflect flash photography you are committing an offence and may find yourself being prosecuted for attempting to pervert the course of justice.
Changing a number or letter or displaying a false number on your motorcycle is a serious offence, which is enforced by customs and excise. This can carry a custodial sentence of up to two years. The police also have a serious sense of humour failure if you have a number plate that can be flipped up and down (although if they cannot prove you used it on the road you should not be convicted of an offence).
Displaying no number at all is a non-endorsable offence (i.e. no points), as things currently stand, but you should expect this to change. However, riding around without a number plate is like having a target on your back and once the police stop you they will look at everything to do with your motorcycle.