It may be stupid (it is stupid) but it isn't actually illegal to loiter outside the ATZ of a Class G airport.
Class G is Class G
If this was not so, every pilot flying around Biggin, or Shoreham, etc, etc, etc, etc (repeat for every Class G airport in the UK) would be prosecuted. And there would be loads and loads of them.
Is it more dangerous to fly right across the instrument approach path of some Class G airport (which could happen some quite suprising distance away - hands up all those who have actually looked up the ILS for Lydd whose DME arc starts somewhere in Finland) which happens all the time, and can be pretty hazardous (see that ILS calibration flight mid-air for one recent example), or flying the ILS right in front or right behind somebody else? Both are pretty stupid.
But the vast majority of plain PPLs have little clue what an instrument approach is, and since they are not marked on the map (some are indicated with the chevrons) the airspace is fair game for all.
That's before one gets onto all those airfields without published approaches, where people fly DIY approaches
The veneer of safety/security represented by ATC in Class G is very thin indeed.
If it was not for UK ATC accounting and other practices, every tower could have a radar screen, for the cost of an internet connection and a laptop. Then, subject to transponder carriage, an ATC clearance to fly the approach might mean something useful.