Can't answer for UK rules, but it could be private, unless it's being paid for, then becomes Aerial Work here in Oz.
Weight creates the downwash, I'd imagine the proverbial hair dryer would be more effective than a Robbo
On a sealed surface, a 206 is fairly effective, depending on the amount of water to be cleared. I often used to dry our local Drag Strip at Calder Park Raceway with a LongRanger, loaded up with pax for more downwash, and a very low hover. As far as footy fields/cricket pitches, we'd really only take some of the surface water off after an hour or so with a JetRanger, but that was usually all the club needed to make the pitch "playable". Once the standing water goes, nature stands a better chance of getting normal evaporation going.