Why not trade with ETH?
because anyone who has ever traded in cryptocurrency markets wanted either to increase his money (fiat) or increase his bitcoins. that is why only the two markets are high volume.
the benefit is that if you make more money, well you are making more money.
but if you make more bitcoin, bitcoin itself rises up every month and you make an additional profit on monthly basic on that.
I meant if he only traded fiat/fiat. Since I interpret "I'm in the UK and have been trading in GBP" as fiat only.
In that case he could have profited more by trading crypto currencies, because the fiat markets doesn't fluctuates as much as the crypto market.