Having worked in the aircraft registration department of the UK CAA 1988-1990, I can answer some of this. The information for the out-of-sequence registrations is correct. However, there is some erroneous information in the original post. The G-BNL* and G-BNW* sequences were batches of in-sequence registrations requested by BA as they gave a unique set of last two letters, eg LA, LB, LC etc, by which the airline easily identifies the aircraft within its fleet. So, at the time there were no blocks of aircraft with those last two letters. "Brand New Large Aircraft" is someone trying to make things fit after the fact.
In-sequence registrations started with G-AAAA (well, actually with G-EAAA, but that's another story) and with the exception of G-BSST for the prototype Concorde were issued in strict sequence. In later decades, airlines (not just BA) could request batches of registrations for their fleets and reserve them in advance, bearing in mind new fleets are usually delivered over a number of years. When out-of-sequence registrations became available in the late 1970s on payment of a fee, this changed things slightly as the marks could now be more personalised. A feature of the UK register is that registration marks, once used, cannot be re-used so remain unique. (G-BJCB was an exception, but this was an admin error.) Many foreign registers allow re-use of marks including the USA and Germany.
G-BUSH mentioned above was part of the first batch of Airbus A320s delivered to BA (originally ordered by BCal) - the G-BUS* sequence being obvious.
The initial BA B757 fleet was in the G-BIK* sequence, except G-BIKE, which was already on a private aircraft.
There is no 'PI' in the G-EUP* sequence as a mark of respect to those lost in the Staines Air Disaster on G-ARPI.
G-BOAC was the first BA Concorde followed by G-BOAA to G-BOAG. When the sequential marks overtook this batch, G-BOAH was issued to a Piper PA28 - on which I made my first solo flight
Lord King had a Beech King Air which was initially registered G-LKOW (Lord King of Wartnaby) but it was felt that shareholders wouldn't like to see their money spent on a 'vanity' registration. So, although the money was already spent - and the fee was just GBP180 or so at the time - it was re-registered as G-BLLC.
That's about all the trivia I can think of right now. I'll get my anorak...