I (think) I know the difference between practise (verb) and practice (non-verb).
However, I am not sure which form I should use in cases like the following ones:
I love practising the guitar.
You can try practising with this.
The question is tagged "British English". This answer addresses British English.
Practice is a noun. Practise is a verb. A participle is a verbal form so would use s, whether that is practised or practising.
Even when the word is a gerund (which functions as a noun), it's identical in form to the present participle so it uses an s.