Yea, the fees are real pain in the ass sometimes.
Fiat balance you see reflects the actual BTC price. So if you bought £10 worth BTC at £3200 it makes sense you have less fiat value now (when BTC cost £3100).
So question is HOW MANY BTCs you have agreed to buy (prior finishing the deal) and that amount you should have received in your wallet (maybe minus the tx fee, but this depends on service you have used)
May I ask what service you have used to purchase BTC? (because the total amount could have been further reduced by withdrawal etc. fees)
Just guesstimate transferring 0.0023 BTC should roughly cost cca. 0.0003 BTC fee.
For actual fees you can use https://bitcoinfees.21.co/ ( fee amount is usually stated in satoshis/byte ). Also I usually check the mempool size ( number of unconfirmed transactions) trying to send BTCs only when it's below 20k-30k transactions ( otherwise it takes too much time to process and forces you to apply higher fee to speed things up).