If your objective is to shoot ISSF then start with it. All NSRA leagues and open championships are shot to ISSF rules. You need to be in an NSRA club with a good target bias and good coaches to learn properly.
I second the Dorset league as a good one to shoot in, but you've just missed the start of the winter leagues.
I would advise that you concentrate on single shot first, when you have reached a standard (90+) then consider 5 shot. The current ISSF and national championship five shot events are shot on the same targets as precision. The only postal 10mt five shot events I'm aware of are Standard Pistol and Sport Pistol leagues run by the Scottish Pistol Association, UK clubs can affiliate.
Since the UK lost its right to own live fire pistols, the events we used to shoot with .22RF and C/F are now still shot at 25 mts but with five shot air pistol, i.e. Standard pistol, and Sport Pistol. The NSRA do have a proportional 25 mt target for 10 mts which is a bit bigger than the standard ISSF air pistol target.
Good shooting
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?