Wooden Spoon mate, G-BOAC was used just the once by something that went quite fast...
G-BJCB was definitely used twice, both HS.125-600s.
If a UK registration was "Not Taken Up" for some reason it's possible to re-use it - look up G-AKUG on G-INFO, it was originally an Avro Tudor, and re-assigned in 1990 to a Luscombe as a period registration. I don't know why the Tudor didn't take up its reg. There are a few like that.
I always feel that Doug Arnold's frustrated Orstralian imports G-MUST and G-SMIT should be deserving of re-issue to suitable airframes... but maybe there's a good reason why not.