Heathrow connection to Paris

Whether you will be able to check bags through will depend on the policy of United. Being a separate ticket and neither BA or AF being in the same airline alliance as UA makes this less likely but maybe not completely impossible. If through checking is permitted, you would connect between terminals airside and just go through security at the departure terminal. If you need to collect bags you will also need to go through immigration. You would then travel to T4 or 5 by the appropriate train with bags, recheck bags at departure terminal and go through security. As to time, at least 3 hours between flights would be the absolute minimum IMO, but you have no protection if late. You face the same issue if continuing by train though, as well as needing to get to central London.

Marco,
That is helpful. Thank you very much.

I've done this BA > Alitalia and you do need to do Passport control, luggage, customs and then check in and security as if you were coming in off the street. Immigration/passport control is the unpredictable element in this mix followed by security lines. I think the three hour estimate is a good prediction. If you can get your bags checked through it would save you a lot of time and trouble. Heathrow actually has a handy guide for connecting flights with checked through baggage that tell you what you'll need to do, where to go and even estimates how much time it will take. http://www.heathrowairport.com/heathrow-airport-guide/flight-connections Good luck,
=Tod

BA has a policy that they won't through-check on separate tickets unless both airlines are in Oneworld . However, John would be subject to United's policy. They used to be more lenient in this regard but these policies seem to tighten progressively.

John, I don't know if this milage ticket can be cancelled or changed, but I would certainly try. United has numerous better options Chicago to Pairs. One is nonstop. Many others transfer through Washington, Newark, and Toronto; all directly from the transfer point to Paris.

Be careful of United planes to Paris that connect through Washington, D.C. We flew one of those home once - the connection was so tight we actually had to jog through the airport to make the 2nd plane. United didn't even call it a "connection" - they labelled it a "change of gauge." The time we did the "land in London, head to Paris" bit, we took the Eurostar. We landed at LHR at 8, went to St. Pancras, used "left-luggage," then went to the British Library, had a good lunch at St. Pancras, then took the 2:00 Eurostar. If the plane had been late, there was still enough contingency to make the connection. A very pleasant experience, and it didn't take much more time than the all planes option....Assuming you are heading for the center of Paris, you will either need to go LHR to CDG, then transport into Paris center OR LHR, then transport into London city center and take the Eurostar to the center of Paris. Option 2 is easier, since you eliminate the possibility of a LHR complication due to a late plane and / or immigration and security slow-downs.

Sorry, John, but I could not disagree more with the above advise. There have been numerous threads on this board replete with problems on the time to get through immigration at Heathrow. Admittedly, I have never experienced such. You also have to be at St Pancras at least 30 minutes prior to departure. I have transferred numerous times through Heathrow with little problem. The only problem arose with a weather related delay in Baltimore and late arrival at LHR. British Air took care of it with ease. Oh, forgot, that was British, not United. However, if they schedule the flights and connections, it is their responsibility to get you through the situation, NO MATTER WHAT!! Your problem with your existing plan and the one offered above is that your are responsible, not the airline.

Thanks for all the suggestions.