I know there are amateur radio licences, as well as those for marine VHF and the like, do any of these actually allow for higher transmit powers on the standard 900Mhz/2.4Ghz/5,8Ghz bands?
I know it’s something that’s not too actively monitored unless you’re being an arse. I do have the FCC mod on my Mini 3 so not policing anyone here, but I’m always on the lookout for new bits to learn and for me, it’s not entirely useless outside the hobby as well.
Ham license doesn’t cover it (if you read the fine print and FAQ). There might be something commercial, but you are probably looking at a 5-6 figure sum to do that.
As you say, do it, but don’t be an arse. I have met absolutely no one doing FPV that runs 25mw. On the RTX side, 50-100mw for ELRS is common (and not too much of a hand warmer/burner). On the VTX side, I know of several people using a full watt on the new HDZero VTX. 200-400mw is pretty common. I think 25mw is still enforced for racing, but outside of that, use common sense. if you are flying out in the middle of nowhere away from people, it’s unlikely anyone will care if you are transmitting at 100mw or probably even 1w. If you are in a city, then its probably going to knock out wifi pretty much everywhere, if you max out power usage.
From personal experience, 50mw on RTX and 200mw on VTX is usable in most situations without annoying other people. But I fly a 2S 85mm quad and not very far.
As ever, YMMV.
Cheers for the advice, I figured that would be the case; otherwise you’d hear more about it. I tend to be flying in pretty open spaces where it shouldn’t cause a problem, but we also have a lot of marine & military traffic here, so it pays to be careful at times.
@PathfinderFPV has pretty much covered it.
There used to be a license you could apply for. It was known as a G9 radio experimenters license, but you had to provide a very strong justification when applying for one. They were usually issued to University research departments and similar private sector interests.
I’ve held an amateur radio license for more than 40years, I also hold a Marine Radio General Certificate, a Department of Transport Radar Maintenance Certificate, HND In Electronics and Telecommunication Systems, and numerous qualifications from Defence and Intelligence Agencies, but I’m still only permitted to use a maximum of 25milliwatts EIRP from my vTx’s. 
Its a while since I’ve looked at this.
Isn’t it because the amateur licence requires the radio to be on the ground? (Unlike the US ham licence).
It is indeed.
Section 2, paragraph 2.1, subsection Cof the a mateur radio license states:
Radio Equipment may not be established in any aircraft or airborne vehicle.
So only transmitters and receivers that conform to the Scientific and Industrial license exempt standard, 25milliwatts for 5.8GHz and 60milliwatts for 2.4GHz (most 2.4GHz radios use a dipole which has a gain of 2.14dB which equates to a power level of 100mW EIRP).