Test buy 2023-04-10 in the UK:

fee: 0.99 pounds, minimum buy: 1.99 pounds
bought 10 pounds, minus 0.99 fee, totalled: 0.00039162 BTC (£8.92) presumably after further fees/spread
bitcoin price on Google on that day: 22,777.54 GBP / BTC
bitcoin transaction fees were about 2.7 BTC on that day

Sending 5 pounds to wallet 12dg2FaiZLp3VzDtLvwPinaKz41TQcEGbs

network fee: 0.00001989 BTC
total bitcoin cost: -0.00023928 BTC
new balance: 15,234 satoshi (39,162 - 23,928).
total spent: £5.45
time est.: about 30 minutes

This worked and I received 21939 satoshis (23928 - 1989) on Electrum on one of the outputs of transaction 1177268091cbeaacbcaac5dc4f6d1774c4ec11b4bcffafa555cd2775eafb954c.

Sending 1 satoshi back! The lowest fee in Electron is 1120 Satoshis targeting 25 blocks (4 hours). Let's do it. Failed, server forbids dust, minimum is 1000 satoshi. OK, sending 1000 satoshi, at 1139 fee.