Depending on where your event is, and if it's a weekend (ie light traffic) it could be worth driving to Putney (30-40mins from City) where there is an enormous slip way good for all states of tide. Trailers can be left easily, car parking is free on Sundays, metered on Saturdays.
If your event is in the East, then probably no good, although Putney to Tower Bridge can easily be done in 20mins by RIB.
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