You can renew or replace your UK adult passport up to 9 months before your current one runs out. It currently takes at least three weeks to get your passport via the post and can take longer between April and September. The Post Office offers a Passport Check & Send Service which expedites the receipt of your new passport, but there is an additional charge for this service. Apply early whenever you can. Further information can be found at https://www.gov.uk/renew-adult-passport