The Portfolio currently displays the percentage profit/loss of each investment (in red and green).
Could it rotate (every 10secs, say) between this and the actual £ profit/loss I’ve made.
It can be difficult to calculate this figure in my head, and understand how the investment has contributed to my portfolio’s overall performance.
Or maybe a simple toggle to switch rather than an automatic rotation? 
I quite like the idea of changing it every few seconds. Not much real estate on a phone and that saves adding a switch
I agree on the not enough real estate however not so sure if the few seconds change is the right way to go - usually this is used to signify a change, especially since you have no control over it.
Perhaps a long press on the stock to reveal the amount in cash ?
Just a normal press on the percentage to switch it would work fine and be quite natural. Switching automatically on a timer would be annoying.