you know, you can just lie about your base. It's not unethical to lie if the only reason they're asking is to use it against you during negotiations.
when interviewing, tell the prospective employer you already have the base you want, and that you're only looking for a small pay bump, or retaining the same pay. New employer is now forced to evaluate your pay on an "absolute" basis (Are they really worth X?), rather than a relative one (am I willing to give them a pay increase of Y?). This eases tensions a lot. Don't let anyone hold you back because your previous employer underpaid you.
Yes, someone in HR might find out when they process your tax slip (P45 in the UK), but:
a) the salary person almost definitely won't be looking out for that, they have no interest in screwing you out of a job, unless it's a very, very small company, and the CEO is also the HR guy and the team lead.
b) they're probably not allowed to tell your manager, legally speaking.
c) they simply won't care.
If they really do find out and kick up a fuzz, who cares? you have a legally binding contract. besides, screw those people, if they thought you were worth the money before, what changed? Nothing, that's what. Besides, you now have proof you're worth that money, so someone else will think that too.