Question says it all really. Based in the UK so mains is 240v and 20 amps. It's a lighting circuit.
Use an appropriately listed and labeled electrical tape instead if you must use tape -- duck tape is an unknown quantity when it comes to insulating properties, and also is more likely to combust if hit with sparks than a proper electrical tape would.
Of course, if you can use heat-shrink tubing or some sort of wire connector (wirenut or push-lock) instead, that's a much better solution -- electrical tape doesn't last the way a proper heat-shrink job does.
Hell no. In fact, don't use duct tape for ANYTHING.
If you're trying to cap the exposed end of a wire, use a wire nut.
If you're using a bare wire for some crazy reason and you want it coated, replace the wire with an insulated one or use heat-shrink tubing.
I am an electrical engineer and I HATE electrical tape. It shifts over time, leaving a sticky mess and possibly exposed wires.
No No No!!!
Duct tape has a metallic skin. Where do you think the shiny metallic look comes from?
OK, maybe some modern, cheaper ones achieve the metallic look with non-metallic chemicals (though powdered metal is a pigment type!) Regardless, when companies formulate duct tape, they are keen on making it look metallic and not caring at all whether it insulates.
As such, expect duct tape to have exceptionally poor insulation breakdown voltage (possibly as low as a couple of volts) since it is not optimized for that or tested for that in any way whatsoever.
Regardless, it's not made to last
In any case, duct tape (in modern incarnation) is simply not made to last. I'm sure you've seen the "product" of old duct-tape work where the adhesive parts have fossilized and the sheeting has become crumbly.
You will need electrical tape on it first: 3M brand, 33+ type electrical tape, is the only way to go.
The current traveling through the wire becomes important at this time too.
The 2 weak spots in the use of electrical tape are that it is easy to puncture on sharp bits, and after some time, the adhesive starts to give up. When that happens, the only thing it will not stick to is itself.
Heat Shrink, magic that it is, can not be used as for electrical insulation on mains. Physical protection, yes - but not Electrical protection.
If the conductors are in a mains/breaker box or similar, you want electrician tape and then rubber splicing tape, which will seal to itself and make a connection that will also resist water, as well as provide some thickness to the wire.
Tape should be wrapped in a spiral, with 2/3 of an overlap in each wrap.
You can heat shrink over the rubber tape, should you choose. Be warned, should the connection heat up in any way, the heat shrink will try to shrink more, and can split, without warning.
EE here. I needed to insulate a 5 VDC circuit board (USB voltage), and people seemed to report partial failures (interference on phone wires) as low as 24 V (AC) from random scouring of forums, perhaps this was because it was AC. I found this question on here. I was out of electrical tape and the stores are closed for coronavirus/COVID-19.
Experiment: Wearing appropriate PPE, I used Gorilla Tape, a less metallic, but equally (more) sticky, possibly flammable, terrible choice around energy sources, and not rated for electrical insulation (should not even be considered to safely do the job). I applied 32 VDC across a 1" section, and also directly across its thickness (to simulate one wire wrap) with a current limiting power supply set to 100 mA or so, with additional short circuit protection in case it fails. There was a blip of 1-2 mA, possibly noise (?), but no current appeared to flow and voltage did not drop from hitting the current limit during the short duration of the experiment on my particular sample of gorilla tape. Your tape formula might be more conductive, have lower breakdown voltage, and there is no assurance that this insulating effect will hold up for any amount of time.
Conclusion: While this worked for 5 VDC/USB voltage in very short term, I did not have an easily accessible way to test mains. A lot more could go wrong there if that fails. Perhaps a 1 A fast blow fuse and a GFCI could be used. Sounds face-shield dangerous. Don't try this at home, especially in an uncontrolled environment without safety precautions and if there is any doubt, you probably don't know what you're doing. EEs can get killed by high voltage too, mains is easily capable of that, though probably won't if your skin is dry with high impedance. Got my shiny Cu alligator clips all sticky/insulative and posted purely for academic/curiosity's sake so you don't have to. For SCIENCE!