You have a few options on how to add money to your GBP account:
If you want to transfer to/from a UK-based GBP account, then this will be almost instant and very cheap using FPS transfer. This will cost £0.18
SWIFT transfer from another GBP account. Incoming SWIFT transfers cost £4.00
Transfer money from your USD account to your nsave USD account via "exchange" on the nsave app. Any currency exchange (FX) fees are live charges and will be shown on the app before you complete the transaction.