It'd be easier to add booms to our fleet of voyager than request a bespoke set of F-35A mods. Benefits to the former include, interop with US/Aus and many other typical-coalition tankers, higher fuel flow, easier to tank when heavy, at night. Suddenly UK tankers become hugely more beneficial to the USAF and USN, not just USN, on Combined ops.
Point is, it's possible and the balance of investment will decide, if indeed it has to.