Okay. That looks like the same itinerary shown on Google Flights at $683, so a large savings. Is the $497 price from Expedia, or is it someone like CheapoAir?
I am unfamiliar with the fares out of Denver, but those do look better than reasonable to uninformed me.
Allow me to point out some negatives (which do not mean that you shouldn't grab those tickets if you're sure of your plans):
Those are Basic Economy tickets, which means you pay substantial fees for checked bags. Do some Googling to find out what you'll pay ahead of time for checked bags, because I think the fee I saw a day or two ago was at least $60 ($120 per person round-trip). Also Google for British Airways' size limits for cabin baggage--especially if some of your folks might try to travel carry-on only. I don't do that and am not up on the current rules, but I know that some airlines now have 15- to 17-lb. weight limits (as well as very tight size limits) for carry-on bags.
Some airlines do not charge for the first checked bag on international flights, though I don't know whether there are any such airlines flying out of Denver. Edited to add: I don't see any well-priced tickets that include a checked bag.
British Airways charges for seat selection (not just in Basic Economy) if you don't wait until check-in time 24 hours before your flight. The website says the fee for "longer international flights" starts at $30, but I think you'll find you pay somewhat more. Persistent Googling may turn up more precise information. You might opt (as I have) to select a seat on the overnight flight and take a chance on the way back. Or you might not pay the (lower) fee for the short flight on the return but pay it for the long flight segment. Otherwise, those seat-selection fees are going to add about $100+ to the cost of each ticket. Adults traveling together sometimes prefer aisles-across seating, and I think snagging that type of seating at check-in time might be difficult.
When I was pricing different options last year, I found BA's seat selection fees to be the highest in the market. It wasn't a great deal of extra cost (maybe $10-15 on a transatlantic segment), but if you're buying tickets for four people, it's a noticeable difference.
I recommend Googling British Airways Basic Economy to be sure there are not other significant differences besides the checked-bag fee. Do you get any food, for example?
Your return trip involves a change at London-Heathrow. The connection isn't especially tight, but lots of people prefer to change planes at airports other than Heathrow and Paris-CDG because delays seem more likely there than at some other European airports. Also, many of us prefer to make our connection in the US so that if we miss it, there will probably be more flights the same day. If you miss a flight out of Heathrow, you might end up waiting till the next day to fly home. That is not likely; I'm just pointing out a potential disadvantage of connecting in Europe on the way home.
None of the above is intended to stop you from buying those tickets. The price seems very good--just perhaps not quite as good as one would assume at first glance. I really hate the way those Basic Economy fares just show up without being flagged until you get farther into the purchasing process. It's hard to make rational comparisons.