Call operators with aircraft around the size you are looking for, off the top of my head AWC,BAC,BWD,FLT,EAC,GIL, ask to speak to someone in the Commercial Department. Prices will depend on aircraft base, day of week and time of year but expect to pay anything between 10-20K total, if you need an urgent idea, or dont want the hassle of ringing round, contact an air charter broker who will indicate prices and likely avail for the whole market, Air Partner in LGW and Hunt & Palmer in LON are two of the largest.
Beware ATOL regulations if pax are paying for the cost of the journey individually, no worries if its a 'sole use' charter.
;-)