Advice on Employee Share Plan

[YBR]

In a couple of weeks I will receive shares in the company I work in, and I'm being bombarded with Info - I think there's a gap in my understanding and I hope MN can help me out.

The shares are going to be handed over in the shape of units in a trust. They are "free" but not free of Tax and NI.
At the same time there is information offering a "Tax free loan" of £300, which seems to be out of nowhere. I have to assume that this is to cover Tax & NI (the cost of the "free" handout).

Nowhere does it say what the tax/NI amount is, or give an example calculation. Almost all the info is high level as it goes out globally; I need to check my understanding about the UK situation.

If the shares are Tax/NI'd based on the shares value the day I receive them, I can estimate this. Shares are listed online in Euros, so I'll also have to factor in exchange rate - is there an official rate to use (I've just used google)?. Then Tax (20% or 40%) and NI (12% or 2%). So there will be a range of different amounts based on individual earnings vs Tax/NI bands, is that right?

I don't get anything like £300. I've checked different combos, not just my own.
What have I missed?