How can I set my invoices into sterling £ and not dollars $

QBSE US version only support single currency (i.e USD). You may consider switching to QBSE UK version.

https:// quickbooks.grsm.io/UK-SE

As another option, explore and open a multi currency business account at TW as the virtual bank and utilize the bank feeds.

https:// prf.hn/l/7gg1NWy