Gatwick into London for 4

Prebook a blackberry car. Just did that today. 52 pounds, a private driver and the 1 1/2 hour drive was fabulous!

http://www.blackberrycars.com/

It's a long slow journey by road to London from Gatwick.

My choice would be the direct Thameslink train from Gatwick to London St Pancras International (NOT the Gatwick Express to Victoria). It takes around an hour, costs £10.40 per person (no need to book ahead.)

St Pancras is about half a mile from your destination so at the station, follow the "taxi" signs and pick up a black cab for the last bit of your journey (probably cost about £6).

Or walk it from St Pancras! Really easy walk, but depends on luggage, jetlag, sense of direction etc.

Checking non-peak groupsave fare for 4 (non rush hours for Thameslink Rail):

Gatwick Airport to London St Pancras
Out: Thu 13 Jul 10:40 am
4 x adult with Groupsave £27.40
Flexible ticket with no time restrictions on when you can travel
Not valid for travel on London Underground services.

For some reason the only way I could bring up the single train service for online ticketing was to put in "Blackfriars" as a via station, else it was forcing a connection (?).

absolutely Thameslink as above - fastest and easiest. No steps required at either end, with lifts and escalators at both ends, and an easy walk to UCL.

Perfect!! Thank you all.