Actually it's totally legit.
Atlantic Flight Training is approved for it and does it through DELTA connection. So you'll actually do the ATPL course through AFT but the flying side is going to be DCA and you'll get a commercial JAA license because you'll do the flying test with a certified CAA examiner.
The instrument rating part is though completed in the U.K. either by first getting the I.R. and then doing a couple of weeks in the U.K. OR you can do the WHOLE IR bit in the U.K.
I suggest you simply contact the folks at AFT/DCA for more information on the matter.
Cheers,
Student @ DCA.