I did some research and gathered all the places in UK so far known to carry cubes:
Robert May
How - Order a catalogue from (broken link removed)
Cost - £22 for a pack of 17 grids
Delivery time - usually 1-3 working days. Mine arrived the day after ordering.
Kleeneze
How - Via the catalogue dropped off at your door. Also look through (broken link removed) for any other information.
Cost - £19.99
Delivery time - 5 working days
Rapid Racking
How - Online at
https://www.rapidracking.com/products.aspx?Grp1Code=4&Grp 2Code=34&Grp3Code=109
Cost - £47.70 for 30 grids
Delivery time - 5 working days
Eden Shop
How - Online at
https://www.edenshop.co.uk/erol.html#2406x0&&
Cost - £17.85 for 16 grids
Delivery time - 1-2 working days
Shelving Direct
How - Online at (broken link removed)
Cost - £19.98 for 16 grids
Delivery time - Order by 3pm and items will be delivered next working day
Screwfix Direct
How - Online at (broken link removed)
Cost - £22.99 for 16 grids
Delivery time - Order before 6pm and delivery next working day.