Good advice from Sporky.
I'd add a few things:
1. John Lewis have a decent guarantee, usually two years.
2. TVs connect to t'internet these days. As-is they are a pretty serious security risk. Get a brand you can trust regarding security updates, and, if you can, partition your LAN to separate the TV from the rest of your computer kit. This means logically, not physically.
3. Get a model that lets you put a password on the settings. I have a Panasonic. Whilst it's good in many ways, both the lack of security updates and the lack of a settings password (or PIN) are BIG deficiencies.
And finally, regarding actually making your choice, ask the assistant to put your favourite ones to exactly the same channel. The ones that change camera shots first have the more powerful processors in them. The laggards have slower CPUs and are more likely to go out of sync (sound/picture) with 'difficult' source material.
Also there are two other easy CPU tests:
1. Get them tuned to a low-quality Freeview source (one of the low-quality movie channels, such as 'Movie Mix' or the Travel Channel). Use a channel with a lot of action on-screen, which was originally filmed/recorded in fairly high quality (no Police dash-cam stuff). Look for the best picture on that material rather than HD off Blu-Ray (any screen can do that fairly satisfactorily). That is testing the decoder - making the best of a bad job.
2. View a Freeview HD channel off-air, ideally an action sport of some sort (Formula One is ideal), The BBC is the best bet for this, for technical reasons. One will probably be significantly better at coping with the action than another.
If that doesn't annoy the assistant in the pre-Christmas rush, you're probably not being demanding enough
Have fun. You probably won't notice the good/bad points once you've got it, anyway. We don't with ours...
E.
