Agreed 'tis a bullet by all appearances, but an odd one.
From the shape you might call it a boattailed spire point. But most spire point bullets are softnosed hunting projectiles and usually short, light and flatbased to allow a high velocity and flat trajectory over shortish ranges with light recoil – for smaller targets, sometimes 'varmints'.
This ain't none o' those things. It's got to weigh about 300 grains, and if those rifling marks (note the left-hand twist) are real – they're not especially clear in either pic – then it's been fired into something providing very light and uniform resistance to bring it to a halt without severe scoring and/or distortion. .303 full metal jackets typically bend double or even snap in two in range sandtraps as they topple and try to turn over under deceleration. Perhaps this particular bullet flew a full trajectory in air and only hit the ground spent with little energy remaining.
Left hand twist is a characteristic of 303 and some other British calibres of late C19 and early C20 – I don't know whether Holland and Holland used it in their .375" Magnum big-game rifles, but I doubt a modern (almost certainly US or perhaps Czech) maker would do so.
Is there anything hard, like tungsten carbide, at the bottom of that saw cut?
Edited By Mick B1 on 05/03/2022 14:00:05