The maximum sustained working pressure for each of the class of pipes is:
Class C - 9 bar
Class D - 12 bar
Class E - 15 bar
There is no Class 10 within BS3505:1986 so I guess that I either have an old copy, it is a class in a different standard, or it is a class given within a particular type of specification.
The current standard I believe is BS EN 1452:2000 which is in 5 parts. Part 1 General, Part 2 Pipes, Part 3 Fittings, Part 4 Valves and Ancillaries, Part 5 Fitness for Purpose. There is a Pipe Series 10 in Part 2 (it specifies pipe based on nominal pressure PN and series S)which is rated to 10 bar for diameters less than 90mm and 12.5 bar for diameters greater than 90mm.
BS3505 was generally replaced by UK water companies with a preference for WIS-31-06 which has two pressure classifications Nominal 8 bar and Nominal 12.5 bar. But I think this has also been superseded.