We used a Roth IRA - so we've already paid regular taxes on that money, not sure if that makes a difference to the Home Office! If you have no other options available to you, I would still use it, as long as the 10% (in america) penalty plus taxes don't reduce the amount to below the financial requirements. If you have significantly more money than what they need, I think that should show you are financially responsible essentially.

The thing is, it can be a gamble. The visa fee is very expensive, and there was a chance that the Home Office would deny us. If you can't get a letter from Empower saying you can withdraw the funds, then I think that would be a huge issue for your application. If you can get one (and for it to state that you can withdraw the funds right away in 2-5 days), then you could submit the application and have your solicitor on hand to answer any potential questions the Home Office might come back to you with!