I would like to be able to purchase tickets in advance for many of the attractions we are visiting in London and for our day trips. Is it best to buy directly from the official websites of each site? I am seeing that websites like viatar tours and other such sites offer tickets but I am so afraid that the tickets will not be legit. Does buying tickets in advance like that save enough time and money to make it worthwhile? If so what are some reputable websites to use? Specific sites include Canterbury Cathedral, Greenwich (cutty shark, royal observatory), Stratford-upon-Avon sites, Bath..... I am sort of stressing out and don't know if I need to or not.
The best art museums there are free! Of course that means huge crowds, pick your times to minimize crowds.
I assume you've researched some guidebooks? They explain a lot that will reduce your pre-trip stress level.
I would always buy attraction tickets from the official website unless the (official!) city tourist office had some sort of combo deal that fit my itinerary and saved a noticeable amount of money. Other sellers may very well be marking up the tickets.
In planning this year's trip to England I noticed modest online discounts for some sights, including the Churchill War Rooms. Those are probably available only on the official websites.
It's usually pretty easy to identify the official website when you Google. I think sometimes it helps a bit to include the word "official" when you search, but the first few results displayed are likely to be advertisements, and you may pay more if you go there. You can usually avoid that by paying attention to the web address.
I have been to Canterbury Cathedral (August 2018) and I didn't have the impression there was any need at all to pre-purchase tickets. If someone has different information, I trust they will educate both of us.
I had no issue in Bath at the two museums I visited on the spur of the momen in August 2017, but I didn't go to the Roman Baths or anything Jane Austen-related.
I would caution you not to go overboard in buying tickets in advance when there's no information to suggest you'll encounter significant ticket lines on-site or save money by doing so. Sometimes the unforeseen happens; trips, or parts of trips, get canceled or postponed. Don't spend money on non-refundable tickets that confer no real benefit.