Is this a mole or a skin tag? (Photo attached).

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And is it safe to try removing it myself tying a piece of cotton thread or dental floss around it? It's fingernail sized.

Looks like a sebhorric wart (spelling?). I had one removed at the doctors pre-covid. Very quick and painless! Not sure I would be brave enough to tie cotton around it!

Don't try and remove that. I'd get it checked.

Looks like sebhorreic keratosis to me, not harmful, but I wouldn't advise the cotton thread thing.

That's not like any skin tag I have and I have a few of them. A skin tag is extra skin, still attached to the rest of your body but it isn't a different colour to the rest of the skin (at least going by my ones anyway). I can wiggle them around.
That photo looks like either a mole or a very raised freckle but either way, as the surface of it looks cracked, I'd get that seen to by a doctor (just for peace of mind) and not tackle it myself.

At that size no. Get tae GP.

Looks like sebbhoraic keratosis to me as well - i had one removed recently but went to a private clinic - cost £350 and took 15 minutes. Was well worth it. Not sure NHS would do it now.