For $3000 you can get a paid business ticket from ORD to LHR (mid-week in October as an example). That business ticket would cost you more than 200,000 miles for an anytime award. Or 115,000 if you manage to scrap together a saver award (good luck with that).
If you pay for the ticket you will get EQDs, EQMs with a class of service multiplier and anywhere from 15,000 to 30,000 RDMs.
I would echo what others say, if you find a saver award you are close to and need the miles maybe not a bad idea - but compare it to the cash price of the ticket plus the other potential benefits like qualifying for status and redeemable miles.