[gambiting]

In UK your employer can see what you've earned before. As explained elsewhere, it's because unless you run your own company you don't have to do your own taxes. Government does it for you, even if you have multiple jobs they will figure out exactly how much you need to pay without you bothering with it. But the downside is that yes, you have to disclose how much you've earned before(there are situations when you can make the figure appear slightly larger or smaller,but it's not very easy).

[jarek]

Might be worth mentioning that the P45 form with previous salary only comes into play once a company has hired you and are doing salary tax paperwork.

So declining to state current salary before receiving an offer is still very much a possibility.