Do I need a branded one from Screwfix at £45 or can I get a knock-off one from Amazon for £15?

Does the amazon one have the big hammer and left handed therblig included?

I’ve only ever used proper helicoil kits.
The inserts on the cheap ones looked just that to me , cheap.
I expect they would’ve ok but when it’s motorbike engine casings or fork clamps then cheap is no good .

Are these any good for you?

https://www.ebay.co.uk/sch/jindinnin/m.html?item=255184742671&hash=item3b6a32290f%3Ag%3AWIAAAOSwgcVbDW3s&rt=nc&_trksid=p2047675.l2562

Helicoil are fine for most jobs,buy proper helicoil.

If its aluminium i use the self tapping carbon steel inserts but you need to take your time (warm up the aluminium) use good tapping oil and make sure you use the correct drill size.

Do I need a branded one from Screwfix at £45 or can I get a knock-off one from Amazon for £15?

Does it depend on how important that thread is?
I had a stripped spark plug thread in an aluminium head – I had a timesert fitted as they seem to have a better rep, especially for stuff that you want to occasionally remove and insert.