I've recently used the Post Office 'Digital Check & Send' service.
https://www.postoffice.co.uk/identity/renew-adult-passport
It costs £16 (on top of the passport fee obvs), BUT for that they take your photo for you, enter all the info from your old passport onto the digital renewal form and send it, post your old passport off for you. All done in about 10 minutes tops while you are there. Takes all the hassle out of it, and for not much more than if you did it all yourself (getting a new photo, postage, envelope, time, hassle). You then get regular update emails &/or texts as the renewal progresses. My application was 19th January. I've got alerts to say application and then old passport received. Highly recommended.