13S3P
3 parallel cells for 36Ah means 12Ah/cell.
Best 18650 cells on the market are at or under 4Ah.
So this offer is BS.
But with such money it's possible to make such a battery using DIY methods.
Good cells alone would cost close to 100 pounds.
Then the rest.
I would say building your own spot welder and making the battery yourself is still cheaper than buying from any legit offer.
Myself I made 11S6P battery using LG MG1 cells. 200PLN for cells, ~150PLN spot welder, ~50PLN rest (plastic frame, metal strips for the battery).