An IP phone is just a digital one using "internet protocol" in the same way as a PC connects on your home network to the internet, using either WiFi (radio) or Ethernet (wires)....
But don't worry about it, as a cordless phone (DECT), or pair of cordless phones will do you, as you say. One of the phones will be a "base station", possibly with an answering machine built-in, that has a phone lead (wire) that usually plugs into the new router that will be needed when you get fibre. The router does a conversion from old analogue phone to digital suitable for internet use (that's the IP bit). The other phone of the pair can be put anywhere in the house that is convenient for you, as it just needs a mains plug nearby for keeping it charged up.
We have 4 cordless phones around the house, so we can always hear one them, andvget recorded missed calls. The audio quality is poor, which a feature o DECT, but it's OK for use once in a while.
We have one shared mobile phone (80 pence per month, Lebara on the Vodafone system), and until we switched provider, we needed the landline phone as there was no indoor mobile signal with the previous provider (EE)!
I think when we get fibre, we'll keep the cordless (DECT) phones even if they charge a bit more for a landline number. Maybe all providers give landline phone capability for free - I don't know. TalkTalk would be off our list of possibles though, no matter the price, as we had a basic broadband fault once with them that turned into a massive PITA, as their customer support was dreadful - they blamed our house, when the actual problem was in the exchange or street box!