If I was going to walk somewhere in the dark from OTP it would be the Angelo hotel. You can see it when you come out of the terminal building. I have stayed at the RIN Airport which was fine but I wouldn't walk there in the dark partly because of the distance, partly because the underfoot surface is uneven with plenty of trip hazards and I'm not that brave/foolhardy; also they have one shuttle an hour so you'd be lucky to get 5 hours actually in the hotel.
I thought the airport lounge was ok. It may have a smoking area perhaps but I didn't notice evidence of smoking in the main lounge area at all. It was a bit light on snacks though the yogurt pot with some kind of topping (in fridge under counter) was to my taste.
I will be taking a checked bag on my next trip so I think my bag and I will be spending some hours landside until check-in opens. Big, big, sigh. I am doubtful about leaving it at the left luggage in LHR as only a couple of hours connection planned and might fail to retrieve it.