Probably not enforceable
I suspect that the £20 fee is there to discourage DOSing the court system because they know ultimately, winning a lawsuit based on shakey evidence and accusations alone is going to be near impossible. Similarly - the setting up of a tribunal (as in most civil offenses in the UK) is an attempt to inject a middle man between the accused and the court - between the accused and justice.
As with any defense, the key is building up a solid paper trail of plausible deniability coupled with attempts to reasonably assist with law enforcement... ergo an immediate response following each accusation - e.g.*:
Dear ISP/MAFIAA boss,
I (run an open wifi network|share my password with all my friends and relatives) and at the time I am accused of downloading "Justin.Bieber.2012.Believe.Tour.XXX.Wicked_Yeah", approximately 12 computers/laptops/devices were attached to my router. Also I should mention that 4 of these IP addresses belong to shared computers that (couple|group|kids) share between them.
I have asked around all my (friends|family|relatives) to tell me what they were doing online at the time of the alleged infringement, but none recall. I have tried to check the router logs, alas they were not switched on at the time.
Is it possible you could assist me in identifying the computer that did the alleged infringing. Here is a list of the 12 IP/MAC Addresses connected to my router at the time. If you could identify which one, I will gladly hand over their details.
Please find enclosed a cheque for £20, I expect that if I receive no response by X deadline, that no action has been taken and it will be refunded to my account.
F U Cox
*IANAL and this is not a real template.
The trick is not to stop you getting disconnected, it's to prove the disconnection was unfounded and allow you to seek compensation - as well as showing up the whole process.