You can get the newspaper publication Motorcycle News that comes out on a Wednesday so you could find any shops close to the part of London that you would be staying. personally, I would get one fabricated in India to the exact dimentions you require made from aluminium, I have had fibreglass boxes in the past and recently had a plastic 'Givi' box on my commuter bike, until the bike fell off it's stand and suprizingly wrecked the box! Good luck....ps liked the photos on your webpage.
'He who laughs last, was too slow to get the joke'
Never confuse the map with the journey.