Can I report them to the police ?
I am in the UK - is there any type of authority that investigates such matters ?
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You could report them to the police however it probably isn't worth your time or theirs unless you know for certain that they are specifically targeting you and their IP location is also in the UK or a country we have the relevant legal agreements with.
If it is just a password login attempt via ssh or something similar it is probably a computer with a virus that has become part of a botnet. If this is the case you may want to switch to key based authentication and move to a different port to make things a little more secure and save your own bandwidth.
The only thing that might be worth considering is reporting them to the abuse department of their ISP. You can find this by doing a whois on the IP.