https://m.ebay.com/itm/Universal-360-Rotation-Magnetic-Mount-Car-Holder-Stand-For-GPS-iPhone-7-7plus-6-/142123417719?_ul=BR&_mwBanner=1#vi__app-cvip-panel
Check this one out, I have a few, magnetic windscreen mount which is very strong, and holds a plus size iphone throughout ‘spirited driving’ also, very cheap.
It is not illegal to mount the phone on the windscreen anywhere it does not obscure your view, ie infront of the steering wheel, but you can mount it in the lower parts, mine is centre mounted.
Using your satnav on your phone is legal if you do not touch the phone whilst driving. So if you pre program your route before you set off and do not touch it unless pulled over safely wth the engine off or handbreak applied, then it is ok.
Unfortunatley, most phones have annoying popups that get in the way of the nav screen!!
Not sure about the law of messing with a dedicated satnav device whilst driving, but is probably similar.