There's a Virgin flight from Edinburgh to LHR that gets in at 12:50 p.m., Terminal 1. I have a 3:00 p.m. flight out of Terminal 1 on United to IAD. Is that enough time to collect baggage and get checked in?
I wouldn't risk it. I'm assuming that you want to buy a separate ticket and you have already booked your United flight.
What I would be most concerned about is a flight delay. If your flight is even slightly delayed, you are going to miss your UA flight and if you are on separate tickets, UA will consider it your fault; they won't have to honor your ticket and you could be required to buy a new ticket home. Even if your flight is on-time, you would really be cutting it close. Personally, I never book a separate plane ticket on the same day as my transatlantic flight. I've been delayed too many times, particularly at LHR. I always come in the night before. However, if you are going to fly the same day, I'd definitely suggest that you take an earlier flight.
I agree with Laura. Not only will you have to collect your luggage and check in, you will need to go through security. The official Heathrow website recommends checking in three hours before a trans-Atlantic flight.
If you want to see exactly what's involved in any flight transfer at Heathrow, their website has an excellent tool. Just go to http://www.heathrowairport.com/heathrow-airport-guide/flight-connections/connection-guides and put in all your details. I agree with the above replies; if you're flying on separate tickets, don't take any chances, unless you don't mind missing your LHR to IAD flight and having to buy a new ticket at the last minute (meaning at full price). Do whatever you have to, to make sure you don't miss your trans-Atlantic flight (and this may involve getting up very early in Edinburgh, or flying to London a day early).
Basic Rule for an Important Event: Have a Plan A and a Plan B. You don't.
Thanks for the replies. I've flown through LHR many times, but it's been about a year and a half and I know things are always changing there. The LHR website is very helpful. Oddly, Virgin told me that I would have plenty of time to make the transfer. I changed my ticket to an earlier flight. Thanks!
Kenneth, if it were a connecting flight (one ticket), 2 hours would be fine because they'd still be obligated to get you to your final destination. Also, your luggage would be checked through so you would not have to leave the secure area. On separate tickets, Virgin is only obligated to get you to London.