There are different formats of paintball. Tournaments / speedball is usually played 5 a-side on a grass/astro field with inflateable bunkers. You start in range of each other so it's a lot more intense. CPPS is the national league, there's also the Southern Paintball League (UK SPL) at Bricket Wood near St Albans and the Cribbs Cup somewhere south-westerly. The UK SPL has a 5-man division that has some fairly new teams playing, and some stronger teams too. The tournament sites are usually open most weekends for you to train, either on your own or against other teams - if you haven't seen or played tournament before it would be a good idea to either visit a tournament to watch, and/or go to a couple of training days as there's a huge difference to the woods.
Ideally you'd want 5-7 players plus 1-2 pit crew at each event. If you have 7 players, the ones not playing that particular game can run the pit (filling up pods of paint during games, cleaning players and helping reload between games, making sure everyone is eating/drinking enough) and collect used pods off the field.
There is also woodsball - which you can play at local walk-on days, which are similar to playing as a rental, but everyone has their own kit, the paint is generally cheaper and the games/objectives are more varied. They tend to be run monthly at various sites across the country so much less travel involved. There are some annual big game weekends (i.e. weekend long games) at various times and sites too.
In tournaments, even in the semi-auto entry divisions, A5s would have a lower rate of fire as they are single-trigger - it would be very difficult to hit 10.5 bps with them! Also, hits to your gun and hopper count as "out shots", A5s are pretty bulky and the cyclone hoppers would be hard to protect. Even using remote lines to have the air tank on your back, they would be big targets.
Not many sites offer CO2 now, high pressure air is cleaner, cheaper and more consistent. A5s can run off CO2, as long as you have the correct air tanks (NEVER use CO2 in an air tank!), not sure about piranhas but would be surprised if they can't.