To travel to the EU/EEA/Switzerland now (which is where most people from the UK take their holidays), you need to first ensure that it will have a minimum of six months validity on it on the date that you'll be travelling AND that it will be less than ten years old. As your passport will probably be stamped, both on entry and on exit, it might also be wise to check that there's sufficient blank space in it for those stamps.
On arrival in an EU country, you might be required to prove that you've got a return ticket (or a ticket for onward travel to a different country). You might also be required to prove that you've got enough money for your stay.
(Obviously, those rules don't apply to entry into Ireland, where a separate travel agreement is in place).
https://www.gov.uk/visit-eu-switzerland-norway-iceland-liechtenstein