Tom wrote a great deal of sense and useful info there....but I would take up one point he wrote...

'Your music does not have to have been published or recorded for you to be eligible for PRS writer membership,
so all UK Mudcatters who write or perform (ergo 'arrange') trad material may join.'

I was 'accepted' for membership only a few months ago - after a handful of attempts over a good few years!
I had been 'turned away' as they did not seem to want accept (I assume) any 'little fish' that would cost more
in admin than they could collect for.
I had to submit PROOF that my 'works' were being broadcast and/or performed over the previous year.
It did feel very much like - 'come back when you're big enough'
and a definite feel of - 'we will decide if and when you pass the test'. - Not a very nice feeling! - but I do understand why they take such a stance.

I DO feel they perform a useful and very neccessary function though, collecting and collating royalties that are rightfully due,
distributing them to their relevant member artists.
That is why (as a performer for several years at clubs, festivals, with airplay on radio and even TV!)
I persevered for so long (and, thankfully, was finally accepted).
So be aware - 'all mudcatters may (attempt to) join' - not all will be readily accepted.
But, if at first you don't succeed....