I noticed the stamp duty problem too, doesn’t make sense to spend £1000 and a short while later it shows a value of £998 at 0.3% in profit 
On the same note, dividends are not reflected in the P&L of a position either. If the price goes up by 5% and I’ve been paid 5% in divis, I expect to see a 10% investment gain/loss, not 5% as it currently shows.
Edit: Just to be clear, the overall portfolio P&L will still reflect the effect of dividends, it’s just the individual shares that will show an increasingly inaccurate picture of how your investment is performing.
As to your second point, I have used a few UK Based platforms over the years and none of them do this. Personally I don’t see the point. If I but 500 SBRY, top up another 300, then I sell 400 … what should I see in a two-position scenario? Tracking the average of a single position is the way I’ve always worked and makes sense to me.