SASless speaks great sense esp thread 593 regarding staying visual at the expense of other 'niceties' ...
Pressure of regulation may have led to making less than optimal flight decisions 400ft would have been easy (and obviously legal in the USA, legal in UK with 'rule interpretation measures')
From the photos the local vis is clearly in excess of 3-4km (perhaps 7km) but the cloud is blotchy, blue sky in places and some lower cloud - to stay in good/easy visual conditions would have required being ruder with height and he may have felt undue pressure not to break too many eggs for his omelet.
When you are trying to compromise between the rules and safety it is sometimes easy to put too much effort into rule obeyance. This was a polite and respectful guy being too polite and respectful - not wanting to dissapoint the punter trying to do his best for everyone - not wanting to abort Batt having got it agreed (not wanting to mess them all about). A great guy trying his best to do his best - 1 frequency change away from pleasing nearly everyone.
You can tell he was trying not to be too 'rude' because he didn't land at the (other) HeliPad which he passed laterally 400ft from at only about 800ft as he joined the river.
His head might have been 75% full of rule mitigation.
I recon the top of the building would have been visible (perhaps just in but visible) and being avoided (adequately) with a very hard to spot, spindly little jib pointing, most unfeasibly, at you with it's mast obscured by the building with no flashing light and very little relative motion with the background town, painted in camouflage black against black streets. (of course a flashing light is visible through 50m vis cloud, if it is turned on)
People talk nonsense about the flight being difficult - it was in beautiful blue sky from Red to Elstree - with a 'look' at Elstree establishing non-viability and an intended return in the blue sky to Red (the unread txt that Red was clear was good and useful info) - the dip into Batt was suggested by a txt from the "Client", declaring Batt as "OPEN" which can be interpreted as open in all respects inc wx.
Knee jerk re-action is politically demanded - although the CAA know that is unlikely to be appropriate, great pressure to 'do something'... ban private pilots I suppose?