Can i use Existing Light fitting to power video doorbell?
This has great potential to become a hokey fix-up DIY bodge, but try not to be tempted into cutting all the corners.
BTW, you said it's &quot;only pulling 24v&quot; which is incorrect. It's pulling 240v, but only 0.2A
That's actually less than half the power consumed by a 60w light bulb, so even if your ring is running a chandelier in the hall too, it's unlikely to be anywhere near maximum capacity, which is 5A.
Right. That settled, onto the bodge…
Is it to building regs? No.
Is it actually going to hurt anything if you do it properly? No.
Would I do it? Yes.
You're going to need to wire it in parallel above the drop to the light switch unless you only want it to work when the light's on*. Don't try to wire it in above the light itself. That's a no-no.
Late edit.
I feel I ought to clarify this, reading back.
The ring main runs round from light to light, not from switch to switch, so directly above the fitting you can get to it - but it's in the porch, so it's not weather-proof enough for a cheap switch-mode power supply. You have to find another segment of the ring indoors to wire from (not the drop to the switch, that's not in the ring.)
End of edit now read on…
Do not place it outdoors, even in a porch, no matter what you box it in. That it has air-vents tells you it needs some airflow, so you cannot box it up tight enough to be placed outside the main building. It will get just enough airflow placed in the ceiling so long as it's placed in free-space.
To get it closer to building regs, if not absolutely to spec, you really ought to put an old-style 5A socket on a joist in the ceiling space &amp; plug it to that - that way anyone who finds the socket in the distant future won't expect it to be on the ring main at 13A. That would also allow you to correctly earth the socket. At a push, put a 13A socket in &amp; label it 5A Lighting.  Use correct boxes at any junction you need to make - no insulating tape, no free-floating terminal block &amp; no solder, thank you very much.
Of course, by the time you get up into the ceiling space, you might find you're close enough to a regular ring main to just run a spur off that, legit, kosher &amp; above-board.
If there was any part of the simple terminology I've used in this answer that you don't understand, get an electrician ;)
*That's kind of a joke… don't do that.