You can buy ready made covers in wood or metal. Or you could make one yourself using wood slats. Just make sure they're not too close together to allow sufficient air flow for the pump to work effectively.

BTW it looks like an awkward position for cleaning the window or accessing the drain which I presume is behind the pump.

Mine is horrid looking but at least it sits behind a close boarded fence and isn't smack up against the house. There is a clearance around it that you need to be aware of. Some sort of creative screen even those steel laser cut patterned ones would hide it. Wood slats as Sarah suggests could look really attractive. This would also allow air through if the gaps were wide enough.

There are companies who actually make and sell them, or you can make your own. Most are n the US but I bet as more people have them, more will be available in the U.K.

HUNA sell several on Amazon, EBay etc.

Thank you all for your suggestions really helpful.

They need servicing every year too so don't get a cover that is difficult to remove.