I'd consider the Hisense B7120 from Argos, its a Roku TV and easy to use. It uses a VA panel. Remote control is small.
If that doesn't suit, for example you want a TV using an IPS panel, go for LG instead. I'd suggest making sure you opt for a model that doesn't have the smart remote, as that can be confusing for older people to use.
If panel type isn't of importance (to me, panel determines picture quality, so is very important) then Samsung TVs are also okay. There smart TV is more traditional than LGs, so maybe easier to use for your Dad.