I founded a profitable sports betting syndicate in 2002, then became a main member of another one in 2004 when some of the initial members of mine packed in. I stopped in 2009 and set up my business, also the aggressive limiting of my betting accounts by the bookies had taken away all the fun.
Betting profits are totally outside the tax system UNLESS they are your sole or main income in which case you are potentially liable to income tax but I am not aware of any "post-internet" cases.
If any syndicate members are overseas things are very different. They must declare their profits in the USA even though they are in breech of the Wire Act to make the profits if they are internet betting. In Australia, NZ and most of Europe successful punters are liable to tax - so don't be overseas!
I had a full-time job whilst beating the bookies so there was never any question of it being my sole or main income, and in any case my best year was only £17k up and my worst year was £3k up - 2008 - hence packed in.
The pros in my teams, though, were making over £70k a year from 2002 to about 2007 when things got a bit hot in terms of aggressive bookie limits. ALL of them took steps to ensure they declared income on their tax returns, hence were bomb-proof against the "sole or main income" threat.
Strategies included using the betting profits to buy a string of rental properties and declare the income from that, running seminars on betting skills and charging £1k per attendee and declaring the income from that, being self-employed IT consultant (albeit one making not much cash) and declaring that.
