I've looked at it the past two years...opted for different trips each time (partially cost, partially because the kid wasn't quite old enough to appreciate international travel yet, I decided to wait)...
Anyhow, based on the planning I did...I was looking at the following itinerary for a 14 day trip...
1 day - Fly into Heathrow (~2400)
6 days in London / Salisbury (do a double decker tour, tower of london tour, tower bridge tour, ride the eye, see a changing of the guard, Hamleys, Stonehenge, Windsor Castle tour...among other things...) (~4000 including meals, lodging, misc transport, tours and spending money)
Take the Eurostar to Paris ($250)
4 days in Paris (Eiffle, Louvre, Notre Dame, Versailles, sightseeing sail on the Seine, etc.) (~3000 including meals, lodging, misc transport, tours and spending money)
5 days - DLP (~$6000 US including dining plan, tickets, spending money, though they run "stay free" specials so I've seen it as low as ~3000, including all the same)
1 day - Fly back to US from Paris (~2400)
The whole trip, with spending money and flights and accounting for exchange rate always came around to something like 15 - 20k US. Mind you, if I got serious about the plan I'd reach out to friends in the UK for the lay of the land, and could probably cut that down significantly, but I like to start the plan expensive, and then whittle down from there...so, that's the analysis you see above. I'm sure it could be done far cheaper.
I looked at Adventures By Disney and a few other sites (including this one, which was an interesting concept -
http://www.discoverenglandtours.co.uk/prices.html )
If you do a split trip that's just not Disney (something I'd recommend, unless you travel to Europe often), if you are both under 35, check this place out.
http://www.contikivacations.com/?source=google