Firstly, you only need to have a rear facing one.
However, unlike a car, van, motorhome etc putting a UK sticker on a bike isn't quite so easy. A standard "Oval UK sticker" for a car is normally way too big for a bike as they are 18 cm wide by 13.3 cm tall. (Read about standard car UK stickers here
The only place you can put one that size would be if you had a large box or a big set of panniers.
Some people use a cars number plate sticker (just the rear, yellow one) which is an OK workaround if you can find somewhere to fit it.
Here though we have two motorcycle specific options you can use.
A) A specially designed for the job, "Motorbike UK Number Plate sticker". These are about 24mm x 54mm see image 1
B) An small oval shaped UK sticker especially for bikes. These are approx. 80mm x 43mm