i_am_sy Is there parking available?
Probably easier (and possibly cheaper) is to park wherever its convenient for the Underground or a London bus and then take the tube to Uxbridge -- use http://journeyplanner.tfl.gov.uk/user/XSLT_TRIP_REQUEST2?language=en to find a good route.
I've added Belmont House to OpenStreetMap. It's a very short (less than 2 minutes) walk from Uxbridge Station:
Distance: 116m. Time: 0:01.
Ascend: 0m. Descend: 0m.
1. Continue 20m
2. Turn right onto Bakers Yard 10m
3. Turn left onto Baker's Road 20m
4. Turn slight left onto Baker's Road 70m
5. Arrive at destination
Directions courtesy of Graphhopper
Or go to https://www.openstreetmap.org/directions?engine=graphhopper_foot&route=51.54643%2C-0.47847%3B51.54741%2C-0.47894#map=19/51.54688/-0.47880 for the map
Hint: from the platform, take the nearest exit, which is somewhere on your right as you pass through the barrier.
keithctaylor Is it easiest to get tube tickets at the station? Or buy in advance?
If you have a contactless credit or debit card, no tickets are needed for any travel within most of London. Just scan your payment card at the barrier at King's Cross (it takes all of 1 second) and again when you leave Uxbridge Station. Then again at each end of your return journey. The fare is calculated automatically, so you pay the least cost for all the journeys you make within a day or a week, without having to work them out in advance. Buses are flat rate -- the only difference is that on buses you scan on entry only.
From Kings Cross, see https://tfl.gov.uk/plan-a-journey/results?JpType=publictransport&InputFrom=King%27s+Cross+%26+St+Pancras+International&From=King%27s+Cross+%26+St+Pancras+International&FromId=1000129&InputTo=51.54741%2C+-0.47894&To=51.54741%2C+-0.47894&ToId=&TimeIs=arriving&Date=20191207&Via=