I was going to say, do check very carefully whatever travel options you go for, as elements of public services in London tend to shut down or reduce in frequency around or over Xmas / New Year. The HEX website may already have services changes for that period, as - I would guess - would the Transport for London (TFL) website, which deals with the underground. There is also the "Connect" train service into Paddington if the HEX is not running.

I had family who arrived a few years back from NYC over the Xmas period and - being completely unprepared - found that their only travel option into London was an incredibly expensive black cab