I have to agree with Merchant Venturer about Plymouth, it competes directly with Exeter and since a great deal of business users live in the South Hams anyway I suggest it is quicker to whiz up the M5 than it is to negotiate Plymouth's traffic.
You can now fly NQY/BRS thanks to Skybus and as MV says we in the far South West can connect to Stansted as well for international services so PLY is not really relevant to us.
My vote would still go to someone operating into Gatwick ( who though is the question) as this connects the South West with the South East and also all those International/US routes which STN and LCY really do not.
Quick route to an AOC - buy a Company that already has one and add the type - now let's think who might be for sale at the right price?
Merchant Venturer - It would appear the Trislander you asked about belonged to a chap who wanted to start schedules ex Bristol but it never happened. Ther was a story that it caught light on its planned demonstration flight in front of the Mayor and all the VIP guests but I do not have any other details?