If the RAF was only to buy the Vickers Valiant, there's no reason to think it wouldn't have been developed as the Vulcan and Victor were.
Vickers proposed a Valiant B.3 to follow the Marks 1 and 2 which would have featured the Mark 2's stronger airframe combined with a more swept wing so it could go faster. British engine development was moving fast at the time so it could have been powered by Olympus or Conways giving it the same push as its successors.
You'd then turn the Mark 1s into tankers and settle back for a long career.
One of the reasons that the B2 was cancelled was the RAF didn't want the firms' engineers distracted when they needed the aircraft in service and bugs ironed out quickly. Yes one B2 flew, but it was cancelled before entering production. And all sorts of variants of the Valiant were on the drawing board - including supersonic version, etc. Just as there were for the Vulcan & Victor. Even though Vulcan and Victor did get updates, most of the planned variants were never built, and those that were for the most part years later.
When the Valiant retired, there were 5 operational squadrons. 3 bomber, 1 tanker, 1 recon. They were all retired at the same time. Making Mark 1s into tankers doesn't save them - it didn't save them in OTL.
BTW It does occur to me there is a way to have a long-serving bomber in RAF service. It would be the F-111. Dennis Healey considered a plan to slash tons of defence programmes in the 60s to get a load of F-111s based at 6 bases spread across the med & the Far East. F-111s served for a longer time for Australia, so why not for the UK?