I thought it was perforated all around, not checked what I have in the
shed.
I suspect Screwfix may not carry re physical size - surprised B&Q
carried it (must be a proper warehouse :-)

Ebay source:
From 80mm 25m at £18 plus delivery to larger sizes, it is a big bulky
item re courier.

Online source:
Drains Direct or similar do the same sort of price, but also carry the
UNperforated version for carrying water elsewhere.

Dig a trench of 1-in-200 fall, line with geotextile fabric, infill
with stones. If the area will see heavy traffic you need to use rigid
perforated pipe to stop it being crushed.