Blue Chip is in many Garden Centre including the Wyevale/Garden Centre Group chain. Mail order plants can be very small.
Personally, I don't like Blue Chip. It isn't blue: it's mauve. It doesn't really look like a proper Buddleja. It is a nematode magnet and mine failed to flower his year.

The Guardian are offering it through their supplier who is J Parker Bulb company. A friend deals with them a lot with no problems so far. http://www.guardiangardencentre.co.uk/plant-0005625-kb-1/buddleia-blue-chip/

Thanks BJ and Scotsgran I shall let you know what I decide or if I get shall let you know.