Filing Extension - the seeming Catch-22

In doing research, it's clear there are a number of ways to get an extension in filing (e.g. Form 2350, Form 4868, etc.) But all these extensions still seem to leave you open to penalties and interest on any money owed if you don't pay by April 15. That is, with an extension, your forms aren't due, but your payment still is.

First, is this correct? If so, is there a way to pay to figure out an estimated amount and pay that on or before April 15 in order to avoid penalties and interest? I really'd like an extension so I can take more time to consult with a tax expert, but don't want to pay any late charges. 

If it's relevant: I'm a US citizen, married, moved to London summer of 2008, filing for the first time.

Cheers - 

No extension extend the time to pay.  You can do as you suggested and pay an estimated amount to minimise P&I.

Hi, I have been here since 2005.
I just looked at my tax booklet, the one the IRS sent me. It is if specifically geared for those living outside the US.  On page 7-8 it says that "you qualify for an automatic extension of time to file without filing form 4868..." if you " live outside the US and Puerto Rico and your main place of business or post of duty is outside the United States or Puerto Rico."
Form 4868 is the extension form of course.
After June 15, then you should file the form if you need a bit more time, 4 months to be exact.
But yes, what you owe is subject to interest and the rest if it is past April 15.

But remember if the UK is your home now, (check the definition of tax home for the time outside the States needed to qualify), you don't pay US tax on income earned over here up to 80,000 dollars. I have yet to pay US tax because I don't make enough.
Don't for get to pay your state tax too, for the half year you where there.

If so, is there a way to pay to figure out an estimated amount and pay that on or before April 15 in order to avoid penalties and interest? I really'd like an extension so I can take more time to consult with a tax expert, but don't want to pay any late charges. 

If it's relevant: I'm a US citizen, married, moved to London summer of 2008, filing for the first time.

Cheers - 

You file a 4868 plus a State extension if relevant and cut checks by April 15, if you think you owe.