Have we established whether you will be using the system for 2 channel music or not?
Also, how fixed is your budget?
If you did want to listen to music, I would suggest a Marantz SR601x or SR701x as that would sound better for music, but pretty much identical to the Denon for movies / tv.
That should've said amplification! You could do it but the 3500 doesn't have enough quality power to drive them properly. I found in the big action scenes the sound became muddled, lost the clarity and sound stage the speakers can produce, still worked though. If you search on here I have discussed my experiences and those of others of adding a power amp. A two channel driving the front pair could be done for as little as £150, @Rambles will tell about behringer ones, and you can go up from there. The improvement in sound quality across the board with a power amp makes it a good upgrade regardless.
If you don't want to go more than 5.1.2 then the 3500 and a power amp will be your best and cheapest route. Power needs are the Catch22 with buying 'bargain' much higher quality, low sensitivity speakers which were originally a couple of tiers above your budget and AVRs just don't provide decent power for them until you are looking at ones that cost several thousand. The end result is well worth it though and you'll be happy for years to come.
Music will be listened to, but not the priority. Some wiggle room in the budget, but I can't really justify spending 3 times as much on the AVR... not at this point in time anyway.
The A800 on top of the 3500 would make more sense, and it seems essential if I have Atmos speakers. I did see the Marantz 6013 for £699, but but's very close to the 3500 specs, just marginally more power, so I'm guessing still not enough juice overall. It does have the extra channels, but I'm not sure I see myself going beyond 5.1.2 anytime soon.