Well at the risk of being pedantic, technically it isn't.
Leaving aside the issue of "British" law (I will confine myself to English law as opposed to Scottish or Northern Irish), then the general principal is that you can do anything that isn't specifically forbidden. As there is no written constitution, there is in general no specific law protecting peoples' rights.
Hence "intellectual discussion" is generally subject to all the normal laws that apply - you can defame someone in intellectual discussion (subject to the usual defences) or run foul of anti-terrorism legislation or race relations law. Whether anyone (including the CPS) would actually prosecute in a given case would depend on a variety of factors, including public policy, but let's not pretend there is an absolute right of free speech in England enshrined in law. There is in the US, enshrined in the constitution, or so I understand it.
As regards copyright law, this is very complex in England (and I am sure in the US as well). As well as copyright, there is also trademark law, and action can also be taken under the old principle of "passing off", where members of the public are likely to be mislead as the the source or provenance of goods etc.
Good grief, what am I doing - this is supposed to be my hobby not my work!
Good luck to anyone producing anything that enhances the BRP experience - hopefully with full approval of all involved.