Firstly I think its great that you are a conshiensis citizen and taking care to find out about the laws. You are correct about most of your comments about general flying, that you can use the IRIS for fun as long a you stay within the required range and don't fly over people etc.., but here is the official rules/guidelines
Small Unmanned Surveillance Aircraft Air Navigation Order 167
(1) The person in charge of a small unmanned surveillance aircraft must not fly the aircraft in any of the circumstances described in paragraph (2) except in accordance with a permission issued by the CAA.
(2) The circumstances referred to in paragraph (1) are:
(a) over or within 150 metres of any congested area;
(b) over or within 150 metres of an organised open-air assembly of more than 1,000 persons;
(c) within 50 metres of any vessel, vehicle or structure which is not under the control of the person in charge of the aircraft; or
(d) subject to paragraphs (3) and (4), within 50 metres of any person.
(3) Subject to paragraph (4), during take-off or landing, a small unmanned surveillance aircraft must not be flown within 30 metres of any person.
(4) Paragraphs (2)(d) and (3) do not apply to the person in charge of the small unmanned surveillance aircraft or a person under the control of the person in charge of the aircraft.
(5) In this article ‘a small unmanned surveillance aircraft’ means a small unmanned aircraft which is equipped to undertake any form of surveillance or data acquisition.
FPV Frequency and Power
Currently in the UK we are limited in power levels for various reasons, for 2.4ghz for example in the UK wifi runs on this, so does bluetooth and of course RC control transmitters. In the UK we are limited to 10mw of transmission power for video use on 2.4ghz so we do not interfere with any of these other services. On 5.8ghz we are limited to 25mw. You can possibly use higher power TX if you want but you need to get permission and they will probably not even listen to you without a radio license.
I personally have not ever gone down this road as I find 25mw if just fine for fun hobby flying. I also know of people who have been using more than 25mw and its usually not a problem assuming you are far away from anything you could interfere with.
However for the official rules you can read IR2030/27 which talks about non broadcast video frequencies you can use without a license.
So to stay legal you should use a 5.8Ghz 25mw transmitter, but I find these give decent video signal to about 500m with cloverleaf antennas. But if you get fancy and use antenna tracking with high gain antennas on your receiver some people have flown to 1.3Km away!
And just a small disclaimer. I think the info I said is correct but I could be wrong!