Those on the UK Beverley Squadron in the UK in the 60’s will recall that the call signs for use only in the UK were allocated by the ‘Siggies’
The Wg Cdr in charge of the Air Transport Development Unit (ATDU) had a very slight speech impediment and his allocated callsign was always ABC or whatever followed by 66 which the said Wg Cdr Shandy S pronounced as schlix schlix

Sadly, the explanation is more earthly - VSEL (Vickers Shipbuilding and Engineering Ltd, latterly part of BAE Systems Marine) to whom the prefix (VSB) and callsign (Vickers) is still allocated.

They had (may still have) a couple of King Air 200s based at Walney Island.

I thought it probably was but the image of a couple of pilots in dog collars still makes me smile! :\:}

Clipper 1 was the westbound service, in line with Pan Am's system, which had odd numbers for all westbound flights. On a slightly related point, I recall being in the USA and hearing Pan Am Express Dash 7s and ATR-42s using "Pan Am" as their callsign.

Incorrect, "Giant" is Atlas Air I believe. They do fly charters for MAC though.

'round the world'
 

Clipper 1 & 2 were not true round the world services as Pan Am had no internal US rights. So Clipper 1 turned round in 'Frisco becoming Clipper 2 and returned New York West bound.

Not just the Bedford Ferry operated by Transport Flight (Devons - not Doves - and occasionally the Dakota and latterly Navajo Chieftains); all RAE Farnborough pilots in Experimental Flying Department (comprising Experimental Flying Squadron [A, B and C Flights], Transport Flight and IAM Flight) had a personal number preceded by 'Nugget' whilst Met Research Flight at Farnborough initially used Nugget but in the late 80s became 'Metman'(MRF).
When the Farnborough Experimental Flying Wing (as it became in the '90s) moved to Boscombe in '94 they became 'Gauntlet' followed by the pilot's personal number.
Dunsfold test pilots used 'Hawker' followed either by a letter (Hawker Delta - Duncan Simpson) or a number (Hawker 97 - John Farley).
One that always puzzled me was 'Bluebell One', usually a Varsity operating out of West Malling; what did it do?

I only remember Raffles One as being a Dove (c1969 - 1971)

When they converted some Sea Vixens to drones at Farnborough, the test pilot used a 'Rushton' callsign, flying up from Tarrant Rushton to either test fly them or ferry them to Llanbedr.

Northeast was the amalgamation of 'BKS' and 'Channel'. I can remember 'BKS Yankee Delta' becoming 'Northeast Yankee Delta', the 'Yankee Delta' being the aircraft's (Trident 2E) 'last two'.

BKS became BAS with Cambrian (BAS british air services) then Northeast

they did not merge with Channel AW - who went bust

Northeast did then obtain the 2 ex Channel Tridents which were YB and YE to join their own YC/YD
YA having gone to air ceylon as Channel did not NTU all 5

YD was w/off in Spain

all the a/c were Trident OneE's not 2e's

Cambrian had a jet fleet of ex channel and autair 1-11 400's

That puzzled me because I have a very vague recollection of it, not sure why: however Google and PPRuNe are your friends, as lo and behold, from this very boutique, namely the Varsity thread...

Here are a few obsolete UK callsigns for the list;Intra Airways "Chanisle", SouthWest Aviation "Skyavia",AB Airlines "Aztec",and finally Air Southwest "Swallow".

What were the two Air Wales' (2000s ATR42s and 1980s Bandeirantes) callsigns? I seem to recall the ATRs using 'Red Dragon' but it might be my imagination!

Airtours used tourjet for a while. Can anyone remember Astraeus's callsign- was something like starjet. Have I mentioned Chalgrove for Martin Baker?

"Flystar" for Astraeus. And "Red Dragon" is correct for Air Wales. "Starjet" was used by Novair, I'm pretty sure. They succeeded Cal-Air which used "Caljet".

“EuroFlite”. Hope it counts as an ‘airline’. Early to mid 1980’s. Scheduled service originally from / to Luton LTN EGGW & Brussels BRU EBBR. Starting with Cessna 404 Titan and graduating to a E110 Bandeirante Eventually also serving East Midlands EMA EGNX

Middle east Airlines used to use 'Cedarjet' do they still? The flight numbers 202 LHR-Beirut are unchanged from the 1960s when it was a B720.

BWIA were Bee Wee or West Indian

i also remember LHR ATC shortening some call signs as well Scandi and Lufty being the ones that stick in my mind.

Defunct because they are no longer around would include
TWA -Transworld
Seaboard and Western -Seaboard
Flying Tiger line-Tiger- very cool
National Airlines (National One being the daily LHR-MIA DC8 later 10 always with a pilot with a southern accent and the WW2 style girls name under the flight deck windows to fit in with their entirely non PC advertising)

Happy days back then with a great variety of aircraft types and unusual airlines operating funny schedules like Iraqi Airways Viscounts only on Sundays (about 6 stops from Baghdad) and Aerolinas Argentinas Comets who I think were also Sunday visitors.

Very different to the endless parade of A320 series and T7s which seem to take up about 75% of movements today

They do indeed.

TNT's 'Nitro' callsign always made me smile.