Can some give me a detailed idea how HoD works?
Sure.
Generally speaking it's arguably the simplest & most reliable way to book a good value punt in London.
So don't let the detail you've asked for make you think it's complicated!
( Granted, there's going to be some reliable indie SP's out there who are just as easy to book & even better value - but once any of those indie's become widely known they're not going to be easy to book, especially for a punter without e.g. an AW acount with lots of +ve feedback. Rarely does any individual SP stay reliable & available for a long length of time. Whereas HoD have been providing a reliable service for London punters for over 20 years. )
I WhatsApp/SMS them with the girl and the time. Then what?
They confirm availability or tell you she's busy & ask if you'd consider alternatives. Sometimes punters have got no answer and found it's better to call by Voice.
If your time is not for a same day booking, they ask you to call or txt again to confirm on the day of the punt. Otherwise you just turn up at the agreed time.
If the hot girl you have in mind is one of the more popular lasses, you probably have to book in advance. (Though sometimes you can book even the most popular lass on the day, if youre lucky to call soon after she has a cancellation.)
How safe is it?
Very safe mate, as a long running business fully in their interests to make it safe for punters.
However - it is a very popular and well known location. If you want the max possible safety, pay a bit more for a booking from one of the well established up market agencies. (HoD is essentially an up market brothel or parlour)
What's the venue like?
Like a good quality bed & breakfast. Fit for purpose - but not lux like you'd get with some agency bookings or even with the posher indie lasses.
Are there any secret cameras about?
Can't say for 100% - I've not bothered checking for Camera's though assumed they'll be there. Other punters have said they have noticed cameras. If that's the case, then IMO it would purely be to protect from criminals - which rarely bother HoD AFAIK, but it must happen occasionally. Decent punters have nothing to fear from any hidden camera's, and it's very likely they delete footage each day except in the very rare case there's been a criminal event.
What's the etiquette like upon arriving?
No particular etiquette needed from a punter. Here's the process. A maid or WG will answer the door and typically show you to a bedroom. (You can take a shower if you like, though most rooms dont have en -suite) The lass you've booked to see will soon arrive in the room and the punt can begin.
Exception: sometimes the bedroom is not yet free & the maid might sometimes ask you to wait for a few mins in a washroom or whatever - they try to avoid punters seeing each other.