What specific notes do you have, and are they paper or polymer (plastic)?
For a 2018 trip, you should be fine with any 5 GBP notes. The 10 GBP note ceased to be legal tender in March of 2018, so you likely do not have paper 10 GBP notes, so those may be OK. However, if you have 20 or 50 GBP notes, those have been withdrawn, and you will need to exchange. Polymer notes (20 and 50) were issued in 2020 and 2021 respectively, and ceased to be legal tender in 2022.
If you have 20 and 50 GBP notes, simply go to a post office first, there are many small ones in WH Smith stores and similar and try to exchange. Otherwise, yeah, the fallback is a BOE, but you need to be there early, and be prepared to waste most of your morning.