Thank you for writing on the Community Forum. 🤗
As you can read from this page
on the official Citizens Information website it should not be a problem given that there are no routine passport controls in operation for UK citizens.
However, you must show identification to board a ferry or an airplane, and some airlines and sea carriers only accept a passport as valid identification. So you should also check with them what type of identification document they require.
You can find the remaining details on the page I have shared with you, but I also advise you to inquire through local authorities about traveling with expired passports and whether this could be a problem.
I hope this information helps you!
If you need any other advice on your itinerary too, don't hesitate to write to us.
Have a great day. ☘️