I think that it's likely you'd fall foul of the 'reward' part of the law (and insurance requirements relating to 'hire and reward').
I recall a case in the newspapers where a regular customer called her local one-man taxi service but the guy was already committed to another job. His wife took the call and, rather than upset the customer, she said that she'd pick her up and take her to her destination. She made it clear that she would make no charge because it would be unlawful for her to do so. Unfortunately, while carrying the passenger, her car was involved in an accident. The taxi driver's wife ended up in court, where it was ruled that simply trying to maintain goodwill for her husband's business constituted 'reward' in law. So accepting a tip (even if it was for charity) might also be regarded in the same way.
Chris