There is nothing illegal about it, see SERA for legal minimal VFR altitudes (the only way to enforce “land clear rule” is to have a restricted area, but for now EGR160 only applies to single engine helicopters except on published routes and reporting points), but the “old land clear rule” may come back when ANO starts to get edited independently from SERA, it’s called (UK) SERA now…
However, it’s reckless and if you crash, you will surely get the reckless clause of ANO but you have to crash first to get that tested in courts…
Anyone who thinks they can glide to good landing sites around from 1400ft when passing overhead Vauxhall Bridge has probably never flown twins and just guessing they can fit a single in Hyde Pk? or flew there in singles but it was 100kts winds at 2kft p ? or flew it in high perfoamce gliders?