As a rule yes they are, they even publish a standard scale of charges.
They do require you to have special "crown indemnity insurance", which you can arrange seperately or buy from them on a per-landing basis.
Assuming you are looking at a Royal Air Force airfield, the best bet is to call the airfield operator and ask to speak to "Squadron Leader Operations" who is the chap in charge of such things and will be able to discuss requirements and make a decision. If it's a Navy airfield, the title is "Lieutenant Commander Operations", who does roughly the same job. SLOPS / LCOps, is usually a serving pilot in a temporary ground postibng (and separate to SATCO, although usually the same rank).