The maintenance programme on the Islander and Trislander is such that anything other than very short sectors is a no-no. Alderney-London (regardless of the airport!) is a complete no-hoper.
TCAS FAN, there is no requirement for security on an aircraft of less than 10t MTOW (which the Islander is by a long way). If commercial services from Lee on Solent were to be launched, they could be flown VFR and with no security, subject to a safety case for both - provided that the airfield remains licensed as it is today. No need for taxpayer involvement!