For the law...go here
DVLA FAQ As for practicality...
As Glyn says,that's Northampton though...pretty much the same here in Notts, however go into, say, Lincs and it would be Goodnight Vienna, a colleague at work got done in that fine County for digits 4mm too small ( height ) but the slightly better news is it does not carry points.
If your bike is a 51 reg or newer then the single line plate on a bike is illegal anyway, from 1.9.01 all bike plates must be two lines.
Getting stopped for a plate just allows closer inspection of various bits.... pipes & visors spring to mind!
Have fun anyway...
Andy