I get there are three lot sizes: How is one converted into another, the value of each and how are they indicated in the "Volume" field in MT4? There are actually four "lot-sizes", if you include nano-lots. But, nano-lots are not nearly as common as the three you referred to. So, let's stick to the three that you asked about. The MT4 platform allows you to use position sizes which are multiples of micro-lots, multiples of mini-lots, or multiples of standard lots. And the MT4 platform shows all of these various position sizes as multiples or fractions of standard lots.
What you cannot do is trade a fraction of a micro-lot say, one-half of one micro-lot. The minimum position size is 1 micro-lot. Your position size 1 mini-lot equates to 10, units of GBP, because GBP is the base currency in the pair you are trading.
Notice that your broker's maximum allowable leverage, and the margin required on this trade, do not figure into the calculation of pip-value. You can calculate pip-values by hand, if you love to spend your time with trivial tasks. But, it's much easier to let your trading platform do it for you. Or, if you want a "second opinion", use a pip-value calculator, such as this one on the Babypips site Pip Calculator: Even after all that, the "exact" pip-value given by the pip-value calculator may not agree with your broker's pip-value; but, it should be close.
Your broker probably rounds the exact figure up or down, as the case may be. All of these calculations are a bit complicated. And they're definitely tedious. So, you didn't exactly get the "simple" answers you were asking for. In this case, you could have a simple answer, or you could have the correct answer. Once that's explained, how is margin equated with lot size to determine pip value? Please make it simple. You want to place a trade with the following position size 1 standard lot the MT4 platform will show your "volume" as 1.
Margin and pip-value are two separate calculations.More...