Watch trade in price disagreement

You should have had an option to reject the revised offer and have the Watch returned to you.

See the following from Apple Trade In - Apple (UK)

Why did I receive a revised trade-in value offer? 

If the condition of your device is different from what you described, we’ll provide a revised value.

If the revised value is lower than the initial estimate, you can either accept or reject it. If you accept it, we’ll continue with the trade-in. If you paid monthly and received an instant credit, we’ll charge the difference in value to the provided payment method. If you paid in full, the revised amount will be credited to your original payment method (or in some cases, via bank transfer). If you reject it, we’ll cancel the trade-in and return your old device.

If the revised value increases, we’ll automatically increase your trade-in credit.