It isn't, but that actually isn't really a problem. The information required to pose and steal a single voter's vote is pretty easy to come by. But it's an absolutely terrible way to steal an election, simply because it doesn't scale well.

While it is relatively simple and probably a low enough risk to steal a single vote, realistically to flip enough votes to guarantee a desired result you would need to do this several hundred or possibly even thousands of times. There are only so many disguises you can use or polling stations you can go to within an election constituency before you get caught. Also, there's the time constraint involved. You need to do all this in the span of 12-18 hours on a single day. An individual cannot manage this by themselves.

So now you need to scale up your operation, so you enlist a whole bunch of people to split the vote stealing with. Now you have a conspiracy which is a huge risk to discovery, and also likely carries a more harsh punishment should you be discovered. Nobody is going to steal an election this way.

It is much easier to steal an election by targeting a later step of the process, either by compromising the integrity of the ballot boxes via corrupting election officials, or in areas where electronic voting takes place (not the UK) manipulating the tabulation of the votes somehow. In countries where democracy is valued, these steps of the process are hardened quite significantly, with multiple safeguards to prevent tampering.