Lady Fiona has put together a very comprehensive document on travel options from the various airports around London to the site – you can download a copy here:
Buckden is located on the Great North Road (now the A1), giving easy access from north and south. It is also located about 5 miles south of the A14 giving easy access from east and west. There is a large car park. Vehicles can be brought into the courtyard for unloading only but will then need to be returned to the car park.
The site is about 5 miles from both Huntingdon and St. Neot’s, both of which have stations on the East Coast Line from London to Peterborough. Taxis are readily available for ~£10 per trip.
The Eurostar arrives in London from which a further train can be caught (see train connections above)
Heathrow and Gatwick: train into London then East Coast line to Huntingdon or St. Neot’s.
Luton: train to King’s Cross/St. Pancras and then East Coast line as above.
Stansted: train to Peterborough and change to East Coast connection to London.
There is ample taxi service from both Huntingdon and St. Neots’ stations.