I use a variety of retailers depending on a few different criteria.
Category 1: Regular or simple journeys
Where I already know what ticket or combination I need, or it is obvious from research on brfares without needing to query a split ticketing engine, then I use:
1(a): a retailer that offers some sort of discount or cashback (e.g. LNER). I used Ticketyboo for the first time recently as there was a one-time £5 discount. This also includes irregular commuting (full-time, 60% office-based), which is the bulk of my spend, for which I don't need a journey planner at all and minimising the cost is important to me.
1(b): I sometimes get Rail Travel Vouchers and will buy from ticket offices for this type of journey in order to get rid of them.
1(c): There are certain ticket offices that I wouldn't want to close, so I sometimes buy from them anyway, if there's no strong reason to buy elsewhere.
Category 2: Long-distance journeys where I need to search for the optimum itinerary and fare:
I normally end up looking on the forum's site for these, and if I look and find a split then I always buy from the forum's site as I agree with
@yorkie that if you don't, it's like browsing for furniture or electrical goods in John Lewis and then buying from Amazon. It's just going to put John Lewis out of business in the end. The forum's site's customer service is at least as good as, if not better than, John Lewis's in my personal experience, which cannot be said of some of the retailers that offer cashback.
But if the forum's site doesn't find a split that saves money, or doesn't find one that I hadn't already thought of and found elsewhere (like splitting at Kings Cross to go north from London, or splitting at Leeds to get to Halifax), then I might use another site either for a discount or because they offer fee-free changes if that's important to me (for example football away days in the winter where postponements are a real possibility). I favour RedSpottedHankey over CrossCountry these days for that, because CrossCountry charges for ToD even on cross-London tickets that cannot be fulfilled as e-tickets.
There are a few more complex journeys involving starting or ending short, non-stop splits involving day rangers or Travelcards, splits with overlapping validity, non-obvious routes etc where other considerations come into it. Ticket offices can be good for some of these, as can buying from a TOC as a way of shutting down any potential argument about validity. Equally, sometimes the forum's site is the best or only option because it allows multiple via points.
So it really is horses for courses. I wouldn't use the same site for everything, because not only do different sites suggest different itineraries and/or unearth different tickets for a given journey and have very different search interfaces, but they also have different levels of aftersales care and different policies on changes and refunds (albeit the refund policies are soon to be levelled down to something worse than the lowest common denominator in many respects - fees will still be a differentiator where refunds or changes are allowed).
None of the above alters my belief that for most passengers, the ideal scenario is to give them no choice whatsoever and just have one state-run retailer. People don't want choice.
Markets generally don't work. If they did, sites that make their money by offering very simple user interfaces that often fail to find the cheapest fares and thus overcharge people, wouldn't be dominant in terms of market share, but they are. In such a scenario, making fares cheaper would then become a political issue, rather than a matter of studying to advanced technical college level in order to work out how to pay much less than the average passenger for the journeys you want to make.