The BoE ones are the "real" government-issued ones. The interesting ones are the genuinely private ones by the Scottish and Northern Irish banks. Including my favourite ever, the Northern Bank portrait-format space shuttle: http://www.polymernotes.com/northireland.html
Of course, in 2004 Northern Bank was acquired by Denmark-based Danske Group. So now, you have Northern Irish banknotes, which are UK pounds (GBP), with Dankse Bank written on them!