AFISO (aerodrome) providing surveillance service using ADSB.
 

Is this permitted these days? Took me a lot of training to get my radar tickets on a full ATCO licence, and needed a unit validation.

No. Aerodrome Flight Information Service Officer (AFISO). They are required to maintain a log of aircraft reported movements in the ATZ and for a limited area in the immediate vicinity. They cannot pass instructions other than in limited circumstances for aircraft on the ground.

They cannot provide surveillance and have no means to do so.

In any case, ADS-B will NOT show every aircraft operating in their area even if they have a survelllance system because none of them show the 'whole' picture of ALL aircraft including those which have no transponder or other electronc gizmo.
AFISOs might be able to access something like FR24 to view a rough 'picture' of local traffic but unless there is a feed of primary radar, you will never get the whole picture and in any case, unless the CAA approve it, they will not be able to use it even as an ATM (Aerodrome Traffic Monitor) .
Area FISOs at Swanwick and Prestwick cannot use any type of surveillance system to provide an ATC service either.

I would never use FR24 for anything other than general interest because it can be totally in error. In the past I’ve checked up on live flights that our company aircraft has been undertaking. It’s been in error by 200kts or more and has shown the aircraft as having landed or not airborne at all and then suddenly appear half way along track.

Those who think they are using it for “accident investigation” purposes etc should be aware!

FR24 (along with a few similar initiatives) is indeed out of the way for serious work. It is an amateur project, basing upon amateur-supplied information. Mind you, it never claims to be reliable or complete, as far as I observe. That said, I find it quite fascinating to follow, and so do my grandsons :)

And indeed, ADSB can never be the basis for any service as long as it is not universally mandatory. Even then, transponders can fail, like all equipment, electronics in particular.

Reminds me of an incident many years ago at Andrewsfield UK.

Called up airfield to say I was downwind and they replied “ Roger land at your own discretion” . Landed the helicopter on the dead side of the runway downwind and hovered across.
Got a lot of earache from the guy on the radio when entering the building but had to remind him he was an information service and not ATC. He thought I was a fixed wing😀

Don't know the history of radio services at Andrewsfield but nowadays it's A/G so saying 'land at your discretion' is verboten.
I used to be an ROCC examiner and I often 'pulled' people on that because CAP 452 specifically says you must not say it when carrying out A/G.