Why currency does not stay in GBP (£) but keeps changing to USD

Most likely the currency is changing when you log in under user which country is USA. Please go to Settings > System > Static Tables > Countries > Edit Country and remove 'Default Currency' value for USA. Also please make sure the pound currency on Settings > Orders > Currencies is checked as:
[v] user can choose this currency in which prices will be shown
[v] all prices shown by default in this currency