The Rules of the British Go Championships  are the definitive reference.
For those of you who do not have time to read all of the rules and just want to know how to win, here is a brief summary:
To start with, if you want to be British Go Champion, you have to be "British". We take that to mean that either you are eligible to hold a British passport, or that you have lived in this country for five years. Then, since the BGA goes to the effort of running the Championship and we would like something in return, all competitors have to be paid-up members of the BGA.
Once you have dealt with these requirements you will have to play some Go. In fact you need to plan ahead a little bit because to win next year's Championship you must qualify this (calendar) year. You can qualify from any of the many tournaments that take place around Europe each year (provided that they are rated on the European rating system). All you need is to play the entire event (that is not to drop out early or just ghost the odd round) and end the event with 1900 or higher. Please note that deliberately entering an event at too high a grade just to get a 1900 rating will not be tolerated.
The British Championship proper comprises three tournaments. First you must win through the Candidates' Tournament. The event is announced in Upcoming Tournaments and also the electronic Newsletter. It is held on the first May bank holiday weekend. All the players who qualified in the year to 31st March play in a six round Swiss-style tournament. The top seven (or possibly sometimes six) players gain a place in stage two. If you want to be on the safe side, I advise you to win five of your games, but if you can only manage four then you may qualify, if you are lucky with the tie break.
Stage two is the Challengers' League. This is an eight-player all-play-all tournament that normally takes place from the Friday to the Monday of the second May Bank Holiday or a later weekend if there is a clash with the WAGC. You and the other six qualifiers are joined by the previous champion. The organiser would be quite pleased if you could beat all seven of your adversaries outright. Then you are the clear winner of the league and progress to the Title Match as the first challenger. If you come second you will also play the Title Match as the second challenger. If there is a tie for first or second place in the league, those involved may have to have a play-off game (on the Monday afternoon) to decide the second challenger.
So now you are in the title match. This is the last hurdle but the biggest. A best of three or five series of games against the other winner from the League. Each player has three hours on the clock for each game. These are some of the longest time limits in any European competition.
Good luck! You only have to win about 15 games of Go from start to finish of the process. What is so difficult about that?
Of course there is more to the Championship than winning. After all, only one person can win each year, but lots of other players take part, play some serious Go, enjoy themselves, and aim to achieve what they themselves consider success.
These rules were amended for the 2006 Championship, to change the way of qualifying for each of the stages, and also the length and timing of some of the stages. In 2008 AGA Rules were adopted.
Until 2006 the Champion did not need to play the Challenger's League and also some of the best players did not play the Candidates'. Since 2000 qualifying was based on the number of wins at certain levels in British tournaments, rather than achieved rating after events as in the new system.
For reference, some old versions of the rules and pages detailing changes have been preserved: