If you are taking a short break to Paris and need to use the car to get to London (sadly we had to), Eurostar use NCP for the St Pancras station car park and this means a minimum £25.00 per day which can dampen the holiday spirit.
We used Lomax car park in Camden Town (15 minutes' walk) - very helpful staff, they will drive you to the station and pick you up on your return and, best of all, it's about £50 for four days.
Kings Cross Parking (www.parkatmyhouse.com/uk/parking/kings-cross) and Eurostar Parking (www.parkatmyhouse.com/uk/parking/eurostar/) from £11 is now available.
We too used Lomax car park for our recent Eurostar journey to Bruges. Can thoroughly recommend it to anyone who needs to use their car to get to the station.
uk.parkatmyhouse.com/search/parking/station/st-pancras-london/