We recently spent a few nights in the Hilton London Paddington, and enjoyed its blend of convenience (it literally is inside Paddington Station and the end point of the Heathrow Express train) and comforts. There are a lot more rooms that you might think, and the layout is a bit Byzantine but the decor is pretty classic 1920s Art Deco (it was the Great Western railways hotel first). The morning breakfast is nothing special and typical Hilton, but it does save time and money if you're an Honors member.
Best of all, the hotel is QUIET. As others have remarked, you'd think that being in a train station would be nerve-wracking, but we didn't hear a single announcement or a train pulling in/out. It's easy to get in and out of the station to all points London (and England, for that matter), and one of the better international Hiltons at which we've stayed.
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