Starting off with three UK airline examples:
- 18 - British European Airways
- 99 - British Caledonian Airways
- 1R - Virgin Atlantic Airways
British European Airways
Was allocated the customer code 18 but never actually ordered any Boeing aircraft. I can only assume this was allocated in anticipation of either 727 or 737 orders that never materialised. BEA instead went British with the HS Trident and BAC One-eleven for its shorthaul narrowbody jet needs. Once BEA merged with BOAC in 1974 the latter's customer code (36) was used for the combined entity, British Airways.
British Caledonian Airways
I was surprised to see BCal had an allocated code and initially thought that like BEA this was in anticipation of orders that never materialised. But it seems that BCal did take delivery of two brand new 707-399C aircraft - G-AVKA (msn19415 / ln 601) and G-AVTW (msn 19767 / ln 659).
It also looks like BCal placed orders for two 737-299 aircraft that were allocated msn 20090 and 20091, but the orders were cancelled before delivery.
Virgin Atlantic Airways
Despite being quite a large 747 operator none of the 747 classics were ordered new by VS. Also, only four of the thirteen 747-400s delivered new from Boeing actually had the Virgin Atlantic customer code of 1R (the others were four 'Q8' birds leased from ILFC and five '43' birds originally ordered by Alitalia but never delivered to AZ).
- G-VAST (msn 28757 / ln 1117) Ladybird
- G-VVROC (msn 32746 / ln 1336) Mustang Sally
- G-VWOW (msn 32745 / ln 1287) Cosmic Girl
- G-VXLG (msn 29405 / ln 1177) Ruby Tuesday