Flying IFR solo outside CAS in 8/8 VMC very probably reduces your capacity to maintain a safe lookout. Which is why practising instrument approaches is regulated accordingly.
Take the case of Manston with its IAPs in Class G airspace on a nice, sunny cloudless day. As the UK AIP states:
Within Class G airspace, regardless of the service being provided, pilots are ultimately responsible for collision avoidance and terrain clearance, and they should consider service provision to be constrained by the unpredictable nature of this environment.
So without a competent observer keeping a safe lookout flying on instruments in such circumstances, or even watching an autopilot doing so, exposes you
and others to collision risk. A Hawk flying VFR ouside the Manston ATZ may quite legitimately cross the ILS approach, but would be rather stupid to do so where the distance from touchdown and transit height would be close to the assumed height expected to be flown by inbound ILS traffic, particularly if not in RT contact with the aerodrome at the time.
There is no need to 'cancel IFR' in order to continue visually. If you don't, it will simply mean that your flight will continue to be regulated accordingly:
The fact that a pilot reports that he is flying in VMC does not in itself constitute cancellation of an IFR FPL. Unless cancellation action is taken, the flight will continue to be regulated in relation to other IFR traffic.
Note 'in relation to other
IFR traffic'. Not VFR!