Please bear with me, I'm a complete beginner.
Welcome 
There's some things you need to know: first, stop using GBP to describe amounts of Bitcoin. You sent a certain amount of Bitcoin, the fiat-value of it has nothing to do with it when you send it.
Second, Bitcoin fees have been very high for a while, and it doesn't look like that's going to change any time soon.
Your "5GBP" transaction was likely about 0.0003BTC, with 0.001BTC in fees. Basically, the transaction was smaller than the fee. More and more services are now setting minimum deposit amounts, because they can't send such a small amount anymore (it would cost them more in fees than it's worth).
My advice: don't make small transactions "as a test". I know this is very beginner-unfriendly, when I started using Bitcoin I could just play around with cents to see how it all works, but unfortunately those times are gone.
If you want to start trading, fund your account at once with as much as you're willing to lose. And double check the address before sending.
