I had a very similar circumstance. Got an email stating that I was getting a payrise (no offical words or anything, just an email saying congratulations you’re getting more money). I handed in my notice a couple of weeks later and they tried to rescind the payrise for my remaining pay.
I phoned ACAS and they told me where I stood and what to say.
In a nutshell, as soon as they put pen to paper, or finger to email send button, they altered your contract. For them to change their mind is an ‘unlawful deduction of earnings’ and therefore a ‘breach of contract’.
I emailed my HR department and they shat themselves and paid me the money.
Don’t trust me though. Phone ACAS and get it from them: 08457 47 47 47
Dave