We will be in London in October and wish to take the Heathrow Connect for our transportation to our hotel, which is a 5 minute walk from Paddington Station. I'm wanting to pay for as much of the trip as I can before we go. I found the Heathrow Connect website to purchase tickets, but it's in pounds. If I charge them on my credit card will there be an exchange fee from pounds to dollars? I was hoping to find a site where I could pay in US dollars.
You won't get the same exchange rates the big banks get when they trade with each other, but you'll get the best consumer rate available when you charge in a foreign currency and have your CC company convert it to dollars. It'll be at whatever the going rate is at the instant your purchase is posted, which may be a few days after the actual transaction. Many foreign locations (eg. hotels) that allow you to charge in dollars will convert from their own currencies at a very poor rate. Generally speaking, if someone overseas offers to charge it in dollars for you, say no.
See my answer and Tami's answer to your question on this thread.
Using your credit card to buy the tickets online is the same as using it there. If you let them charge you $US it would be the same as letting a hotel or restaurant do that and would almost certainly result in a poorer exhange rate.
I was in London a few weeks ago and bought tickets for the Heathrow Connect moments before getting on the train. It's fast easy and less of a hassle this way.
I bought my ticket for the Heathrow Connect about 20 minutes before I got on it. No lines. You have to take a minute to find out what platform it will be arriving at anyhow. Usually its at platform 12 but it can change.