You are saying that you have nothing written down and agreed before moving job and all of this was done verbally. Speak to your manager again explaining that the offer you have received is not what you understood it was and you only accepted on the basis of the £50k you agreed. If this has no impact then there might be the avenue of escalating this through HR
If the manager won't change the offer (or even if he does) then you have learnt something valuable about them and have to decide whether you can continue to work in this company.
The answer may have to be to look elsewhere and I am sorry to say you have learnt a valuable lesson. A verbal contract is worth the paper it is written on
Thinking on this further, it seems very unusual that a transfer in a company was done totally verbally. It is possible there is a documentary trail that you could use the GDPR to ask for all data relating to you and there may be something in there about the offer but this is a bit of a nuclear button option and risks damaging the relationship between you and your company totally