When I was employed in a business that spent a lot of time (in helos) operating from around cities and towns it was understood that green areas contained within them are not uncongested areas no matter their size. We'd tried to posit this exact same scenario - park hopping across London in single and were firmly told it wasn't on. A large park just inside a connurbation was still part of the congested area. Furthermore, upon quizzing Flight Ops Inspectors their opinion was that even a rural golf course was probably a congested area as it is used for public leisure purposes which surprised us all. The bottom line was that if it isn't farmland, undeveloped or open countryside it's probably 'congested' for the purposes of the CAA who steadfastly refuse to define what congested actually means. (I got busted for breaking the 1500ft rule while conducting a photo op over a disused airfield on the edge of a town under an exemption from the 500ft rule doen to 200ft. Go figure!). My CAA-experienced barrister said it was a waste of time trying to defend the charge as the CAA aways win - perhaps a bit of an exaggeration - but the message was clear, courts are overawed by the 'might' of the CAA and their legal teams and bow to their undefined definitions in blind deference. Anyone who imagines they can exploit what they regard as a 'loophole' in the regs by park-hopping like this is a naive fool and chancing their arm. The CAA are neither that naive nor that dim.
Of course all pilots know (don't they?) that the legality of a flight regarding low flying rulesor the suitability of the aircradft flown is nothing to do with ATC who can and often will approve routings that cross unsuitable areas. If the pilot chooses to fly that route it's up to him. The CAA however, if made aware, will throw the book at you.