A 3 foot void under a suspended floor is quite normal, Barry.
Do read DaveBro's link to Nu-Heat. Nu-Heat are probably the market leader for U/F heating systems. I got my entire system from them and installed it myself, (air-source heat pump/underfloor heating, and unvented hot water cylinder.)
They'll assess the property and draw up a complete system. I'd never done one before, so I was well pleased that they were only a phone call away for any help I needed in installing.
I have solid floors... a much cheaper system.
Suspended timber means you need to fit their "metal heat-spreader plates" first. Those things make it more costly.
I won't go into all the fitting details here because it's all in their detailed guides.
Yes, the insulation is fitted first. Usually between the joists. Then the spreader plates. Then the pipework, and the finished floor.
Usually "Celotex" or "Kingspan" board insulation.
That's for a "wet" system. A dry, electric system could be used. Possibly cheaper to install, but would still need the same insulation.
I guess an electric system would cost more to run though.
A wet system can run from any regular boiler, just like radiators.
Don't worry about heat losses in the void. With 3 feet to spare, you can put in as much insulation as you can afford.