Don't have it to hand now but there is a relevant figure published.
Basically ATS are expected to tell you of NOTAM/Sigmets/Warnings which are issued after a certain time prior to your departure (it could be as little as 30 minutes). Before that time you are expected to have checked.
On a practical basis, in the end it will come down to you making a "reasonable" attempt to obtain a briefing. In these days of mobile telephones, you could get a brief before leaving the house and then ring AIS for an update prior to departure.
If you are departing from the absolute middle of nowhere then perhaps no recent briefing would reasonably be the best you could do (but you would have to prove that).
Airfields I think are required to have adequate facilities........your local farmers field won't have a FBU......this could be taken as having less than the required facilities and thus you could be asked whay you did not make appropriate arrangements (carrier pigeon
) before starting to operate from there.
Regards,
DFC