Hi Maggie,
From North Yorkshire myself (Selby) and although not done eurostar yet i have been to London on the train many times.
Kings Cross and St Pancras and are right next door to each other and linked, just follow the signs to the right as you get off your train, 5 minute walk and don't need to even go outside.
However hotels in the area are a mixed bag and suggest you read reviews before booking, I use expedia a lot and find reviews quite accurate. London hotels arn't cheap and if the price is low its often for a reason.
Mick