Reddit mentions of Pentel Ain Stein Mechanical Pencil Lead, 0.5mm HB, 40 Leads (C275-HB)

Just buy a mechanical pencil and 40 refills for a grand total of £6.12

[Pentel P200 0.5 mm Mechanical Pencil Lead with Black Barrel] (https://www.amazon.co.uk/dp/B002HN26F0/ref=cm_sw_r_cp_apa_X9PDzbPSY99HR)

[Pentel Ain Stein pencil Lead – 0.5 mm – HB – C275 – HBO or 40 Leads] (https://www.amazon.co.uk/dp/B004NNPWCG/ref=cm_sw_r_cp_apa_Z-PDzbJBPXATB)

EDIT:

I am assuming your calculations are for HB pencils, you can easily reduce the number of pencils used by opting for a slightly harder lead like 2H. It is not as smooth to write with as a soft B or 2B but will definitely last much longer.

EDIT 2:

Woodcase pencils are less efficient because sharpening also reduces the amount of writing graphite. Sharpening is not necessary for mechanical pencils.

EDIT 3:

Ammendment to EDIT 2: 2mm Lead Clutch pencils are technically mechanical pencils and do require sharpening. I should have clarified I was referring to the standard sizes of 0.3, 0.5, 0.7, 0.9 (mm) mechanical pencils.

EDIT 4:

I only included links to Pentel pencil and lead, I am not shilling for Pentel. Other brands are available.