From the UK Open Record list:
Free Distance 827.9km T J Wills LS-6 29/05/1986

OK it only started in the UK, finished in France.

At 13:33 09 April 2014, Nigel Pocock wrote:
I make that about 850km wind assisted. Nice flight. We cant do that sort
o
thing in the uk, the landmass isnt big enough!
I pitty the poor retrieve crew - he is probably still driving.
Well done.