Its a problem that is for sure, we are at the time when 2017 models are costing a lot more than their 2016 counterparts, whilst they are not usually worth the extra.
At the same time, 2016 models are hard to find. The 58DX802 you found at John Lewis for example isn't a good buy, but it is still a lot better value than 2017 models which are very overpriced right now.
The Same TV can be found as a refurb on Panasonics eBay refurb store for £800 or even cheaper manufacturer refurbished elsewhere on eBay. Since you want a long warranty though you are going to be limited by the choice of what stores like Richer Sounds and John Lewis carry. Perhaps they have open box models on Richer's clearance section with full warranty? not sure.
So the best thing to do really if you can't find a 2016 model with the warranty you want is just to wait. You will be picking up a lot more TV for your money by doing just that. Right now that 65MU6400 is £1500. Not long after Black friday last year you could pick up the same size 65KS7000 for the same money. The KS7000 is a much, much better TV. 100/120hz panel, UHD premium accreditation for a decent HDR experience and at least some form of local dimming to make those big brights stand out from the deep blacks.
Looking at tv's now, I wouldn't be surprised to see TVs like the 2017 Sony 65XE9005 or Samsung 65QF/C come in to that kind of price range over time.
As for Hisense, the only poor things I have heard is regarding faults in a batch of the 55M7000 model. They are great TV's for the money and really right now obtaining one even as a stop gap until you can buy at the right time in a few years would be better. The 65M7000 is a better tv than the Samsung MU6400 for example and it was available for less than 1k not so long ago, not sure if you can still find one though.
To give you an idea of why a 2017 model like the MU6400 isn't a good buy right now, have a look at the price of last years model over time. If you did want to buy from Samsungs budget line, look at the 2016 KU6400 instead, its £500 less!
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So, continue to see if you can find a 2016 model at a good price or wait until the new models go down in price to get best value!