Last site paint only tourney in the UK I can remember was Mayhem about 6 or 7 years ago, maybe even longer; it's only rec-ball or rental here that's usually site paint only.
And I wasn't talking about fully sponsored teams. The best the majority of teams can get is maybe some free paint on a per tourney or per week/month/year basis then reduced price for anything over that. Even 1 free box per tourney & savings of a couple of quid make a difference to lower level teams & there are quite a few on that kind of deal - very, very few indeed on all paint free type sponsorship, only the top pros. The contracts for any type of sponsorship can be very explicit and any infringement at all even by just one person can lead to the whole team losing their deal - would YOU be willing to give that up for the sake of 1 or 2 tourneys? I know that we certainly wouldn't be allowed to play at a Zap only tourney.
The M25 league needs to be aimed at any level teams, otherwise there can be no national finals against the best from leagues like the Midland Masters or the 2002 Series who do have pro teams entered.